Without a breakdown of vehicle costs it's hard to know exactly why they haven't increased range. Out of PURE speculation, I think it's due to lack of competition. If you get a better battery then you can reduce manufacturing cost, improve reliability, and reduce weight if you don't increase the range. Without competition, there is no reason to increase range. Especially if their charging infrastructure is lightyears ahead of everyone.
My guess is that its for the same reason phones battery life hasn't really changed much over the years: the current range is enough for most people. 300 ish miles is enough for local driving for a few days without worrying about recharging, and enough that most people will need to take a break when driving long distance more often than the car needs recharging.
One, they have increased to some extent. The base Model 3 was announced as a $35k vehicle with 215 miles of range. Today, the closest trim to that has risen in price from ~38k a year ago to ~45k today, but also to 270 miles. The LR was RWD at launch and ~325 miles. It has since grown to AWD and ~350 miles.
Beyond that, they've really focused on margin expansion more than range expansion. The current LR vehicles are remarkably practical for the majority of their use case, especially as the supercharger network has continued to expand to allow charging at the most optimum times in most cases. They seem to have a realization that consumers will tend to choose the highest range, even when its past the real needs for their use case and have slowed range expansion as a result.
They aren't totally wrong. I do think they will resume expanding range once competitors really start to push them to do it. I think that situation is still 2-3 years away, though.
Because we are in the incremental improvement phase of lithium ion batteries. Until a new chemistry/technology get developed we probably won't see giant increases in range.
It does increase slightly. There have been a handful of OTA updates that eek out 10-15 more miles of range, but those improve software efficiencies or better temperature management.