It's because of the direct effects; chlorine gas for instance will almost instantly blind anyone exposed to it, and tear gas can also be fatal. My great-grandfather was gassed in the First World War and only narrowly survived. Chemical weapons were technically already banned by this point, but it was WW1 that prompted the modern Geneva Protocol (not the Geneva Conventions; these are slight different). Unfortunately, none of the Geneva treaties cover their use outside of wartime.
Many people don't realize this but many of the banned weapons are ones with a decent chance of maiming people but leaving them alive. This is worse for everyone involved in a war (everyone here meaning politicians and generals, not soldiers) than killing them. Cluster munitions, flamethrowers, and anti-personnel mines all fit in this category, too.
AIUI they are mainly banned because they could lead to escalations in chemical weapons usage. If your enemy uses tear gas vs cs gas, it could be hard to tell right away and you might feel pressure to use all the tools you have available (including lethal chemical weapons) vs. Play by the rules.
Of course if you are fighting a real war, there is probably going to be chem weapons used. It happened in Syria. It is happening in Ukraine. It will keep happening. Geneva convention is wishful thinking.
something to consider is that in the Gulf War (Operation Desert Storm) the military used a bunker buster on a Sarin gas storage facility and shot the sarin high into the atmosphere, where it then floated far downwind and landed on US troops. Ofc, reporting on it doesn't really consider Iraqi civilians and is only weepy about US soldiers.
Murder is generally punished. Unless you spew pollutants with known biological harm leading to numerous cancerous deaths and cover it up and pay off enough politicians. Then it is generally rewarded or at least tolerated.
Some weapons are intrinsically forbidden because of their effects on individuals: soft-point bullets for instance. These are as discriminate as you want them to be, but are nonetheless prohibited in conflicts. Thus it's not just indiscriminate weapons that are banned by international agreement!
When asked why tear gas is banned, the answer I gave was "the weapon is indiscriminate and chemical" not "all banned weapons are indiscriminate and chemical".
For example lasers used to blind soldiers are generally banned as well and as you point out, lasers are basically arbitrarily discriminate.
Most of Geneva Convention items are things that are huge liabilities to the own sides. e.g., there were such chemical gases that react with gas mask filters so to specifically bypass filtering. No one in Europe wants to pay welfare costs for factory leaks or downwind collateral damages in neighboring countries or army of veterans maimed with that thing, but they will have to if their enemies would use it to their advantages. Agreeing to a universal ban solves that problem.