Why is Lex Luthor bald?

Kryptonite caused it one continuity but dc changes shit every other month so who knows now.

He's so intelligent that his bulging braincells pushed all the hair out from the inside.

The question isn't "why is he bald?", it's "why is he STILL bald?" He's one of the smartest, richest people on the planet, he could cure baldness in an hour.

an artist's mistake

Probably the same reason his metahuman injections didn't work on him. God hates him.

He has, he cloned himself to have a younger, better body, and he still went bald.

Its a reminder of what he lost, Kinda like how Doom keeps his ground beef face

Best Doom is one that just has a tiny scar because he can't handle the slightest imperfection. Likewise Luthor being bald is best when its just an imperfection instead of some comical supervillain accident. He's human, he went bald. He ages, Superman wont. It all ties in to his primary motivation.

Someone should have told Doom that chicks dig scars.

The problem with tying Luthor too hard to his primary motivation is that at his core he's right, which is dangerous for a super-villain to be.