I was working at an AI startup, and I saw our CTO lie in a demo to potential customers

"I quit later, as it became increasingly clear to me that this guy knew nothing about technology, didn't care, but also had a fragile ego where he had to present himself to the company as being in charge, even though he was the worst person for the role. To top it all off, it also dawned on me over time that we basically had an absentee CEO who was working only ~15 hours a week at most. Then when I quit I found out there was a third co-founder who owned a huge stake of the company and I did not even know existed." This is so funny, because describes like the last four CEOs and companies I worked for :-) Is there an alternate reality I have yet to experience?

That sounds like a tough realization to come to, and has been an opposite of my experiences in startups (as a non founder). Ive sought them out because the leadership cares so freaking much about what they are doing, and really genuinely wants to succeed. That can be intense, but at least you know they care. If you are early enough to have both equity and a solid chance at impacting the possibility of success you can have both mercenary and personal incentives aligned in a way that is impossible in a bigger company IMO. I hope you give startups another go if that sounds good to you, and as you say, it sounds like you will be able to see the things you dont want at least. Good luck.