I can think of a couple more major disadvantages they have.
3. Living underwater. Fire is the easiest way to extract energy from raw materials, and there's no real substitute. On land, there's a lot of local, controllable dynamism, but when you put things down they tend to stay where you put them, at least on short timescales. Water is exactly the opposite: lots of changes you can't control, but no way to get a lot of energy all in one place. Dolphins would have a hard time developing technology for the same reason, even if they had the dexterity.
4. Not being apex predators. This is part of the short lifespan problem, but I think it goes beyond that. Not many animals are going to mess with a human if they can help it, which means that it didn't take much for us to get to the point of having some free brain cycles to spend on improving things. An octopus is comparatively small and squishy, and shares an environment with comparatively more large and toothy carnivores, which means that even when they do manage to survive for more than a couple years they're doing it by spending most of their time eating and hiding.
3 seems to be the an argument from the anthropic principle. Oxidation happens in water too, in different forms. Perhaps not as rapid. I would imagine someone from Mon Calamari would have came up with a completely "easiest" way to extract energy.
Slow oxidation underwater isn't useful for anything, though. You can't use it to do work without a lot of additional technology to capture energy over long time periods.
Set aside what we know about human technological progress. You need some energy source to be the base of your technology pyramid, and it needs a few properties: it needs to be naturally occurring so that you can discover it by accident; it needs to be controllable or predictable enough that you can use it selectively; and it needs to be fast/intense enough that you can transform materials without massive time investment. What do you pick? On land, you have fire, flowing water, and, at a stretch, the muscles of large herbivores. Underwater, you have... ???
Also, that's not what "anthropic principle" means.
I think that lifespan it is their biggest disadvantage, and the reason why there is no octopus civilization, unless you want to go beyond the Mountains of Madness.
Modern humans for instance spend more time learning than the lifespan of most animals. If we were limited to a lifespan of 20 years, which is typical for a mammal of our weight class, human society would have been very different. Make it 5 years and there probably wouldn't have been any society at all.
This, I believe, makes them even weirder. With so much intelligence, why didn't they evolve longevity as a way to capitalize on their experience? Why didn't they develop collective strategies that are so effective in other animal species?
Answer to both of your questions is that there was no evolutionary path.
Evolution is blind and path dependent. It only responds to differential pressures affecting just now. Evolution is like
greedy search https://en.wikipedia.org/wiki/Greedy_algorithm
They didn't develop longevity because intelligence + longlivety does not provide immediate advantage.
They didn't develop collective strategies because they the path path to collective strategies providing gains is too long or too unlikely to happen.
Most likely because they have complex tentacles that need fine motor control. Their brains developed to control their body. Their eyes seem to be good as well.
Even in human brain huge area of brain dedicated into hands. Human fine motor skill (or dexterity) is superior compared to other apes. We can do small detailed moves. Other apes and monkeys are clumsy.
After the complexity of brain developed to control their dexterity, octopus gets benefit from spatiotemporal intelligence to exploit tentacles in hunting and moving. It's not surprising that intelligence plan and solve problems as well.
I guess that's it. Some other asocial animals are pretty smart, because they need it for hunting, or something. (Although even the most anti-social mammals still interact with their mom!)
The marginal intelligence point makes survival to reproduce more likely, so octopi that are marginally smarter tend to be slightly more likely to reproduce.
However, they seem to die very soon after mating for some reason related to their evolutionary history. There's no way for a marginally longer-lived octopus to be more successful at reproduction, because reproduction is a one-shot event for them. If anything there's pressure to reproduce (and die) at a younger age, since these octopi would be more successful.
Such are the tragedies of evolution, the blind idiot god.
They live in complex environments. Often on the boundary between water and land. And some octopuses, do seem to make good use their intelligence - for example the mimic octopus.
I guess an important thing is that they hunt in this environment (in which clams also do just fine). And, like us, they don't have much bodily defence against being eaten by others.
> intelligence + longlivety does not provide immediate advantage
You gotta wonder what sort of environmental pressures make intelligent animals less fit for survival by creating communities. Few predators? Very simple environments that don't require passing on information to future generations? High competition for resources?
Energy and nutritional requirements, longer time before reaching adulthood. For humans one of the limiting factor is the female pelvis. Births become more difficult.
A society of monks living mostly alone would be more advanced than a society of busy workers competing for resources in a city hive. Octopuses may also have other means of communication: just like we use radiowaves and electricity, octopuses may have some natural radiowave emitters and receivers in their brains. And a short lifespan is a subjective measure: what matters is the amount of experience.
3. Living underwater. Fire is the easiest way to extract energy from raw materials, and there's no real substitute. On land, there's a lot of local, controllable dynamism, but when you put things down they tend to stay where you put them, at least on short timescales. Water is exactly the opposite: lots of changes you can't control, but no way to get a lot of energy all in one place. Dolphins would have a hard time developing technology for the same reason, even if they had the dexterity.
4. Not being apex predators. This is part of the short lifespan problem, but I think it goes beyond that. Not many animals are going to mess with a human if they can help it, which means that it didn't take much for us to get to the point of having some free brain cycles to spend on improving things. An octopus is comparatively small and squishy, and shares an environment with comparatively more large and toothy carnivores, which means that even when they do manage to survive for more than a couple years they're doing it by spending most of their time eating and hiding.