Why do most players not build Infinity Edge with Pantheon?

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.)

It's very expensive for one, and considering how many times you hit with any abilities, Black Cleaver will generally end up providing more damage on top of the CDR and slight health it gives.