Since he gets a 100% crit chance for his AA and Q through the E passive.
Why do most players not build Infinity Edge with him? (probuilds)
this would increase the crit damage by 50%.
Mainly because Pantheon is an Assassin. You are not really an auto attacker but rather a burster. The main problem with IE is that it increases your crit chance which is pretty much a completely wasted stat on Pantheon. You gain 100% crit if a target is low-life and your spear will also do more damage. 125% crit won't really help (or work) and your Spear cannot crit from normal crit chance.
Another thing to note: You are meele. This means you are basically dead if you get CC'd as glasscannon. Your main job in a teamfight is: Kill the most squishy target and then try to deal as much damage as possible while also tanking a lot of stuff. It's a lot safer if you just grab 1 or two offensive items like Black Cleaver (Scales perfectly with your E) or Hydra (Aoe Damage and more sustain during teamfights) and go tanky afterwards. If you have 2 Items you can basically 1shot the enemy ADC during midgame (and this is where Panth is the strongest).
The only time you could actually buy IE is if you decide to go full glasscannon but as already said I don't recommend it in a real game. You can get focussed far too quickly.
In regards to the provided pro builds, they build Pantheon as a tank. Sticking an IE(Infinity Edge) in the build would go against their role as a tank. Also, sure the extra crit damage would be good for the passive from his E (100% crit chance on enemies below 15% hp) it just isn't as efficient as other items on Pantheon. If a champion is 15% hp or less, it would only typically need 1 or 2 stabs or a spear to finish them off in most circumstances.
Pantheon is not really a 'full offensive' kind of champion. You almost definitely want to build him as a tank or off-tank. Items like Black Cleaver and Maw are great because they a good defensive and offensive items. Hydra is also a good choice because it vastly improves his AOE ability as well as the added sustain from the lifesteal.
At the end of the day builds are personal preference. If you want to try IE on Pantheon go for it, I definetely wouldn't build it first though. Pantheon is also not great as a squishy champion, he has a couple of engages but no kind of escapes or utility to help escape bad situations. This means he can be easily focused down and be almost useless during a team fight.
Pantheon is in the category of AD Casters. You won't be auto attacking with him too much. He does the majority of his damage from using abilities and so giving him crit chance won't really give him much damage. The best way to scale his damage is to build high Flat AD and armor pen.
The role of AD casters is typically to assassinate targets with your abilities. (You could build him as a tankbot with cleaver in order to provide targeted cc and shred armor of tanks if you prefer but it's not usually the normal role. In neither of these roles does adding crit lend you anyhthing.)