Probably mostly a lack of caring. Followed by personal struggles (you said mental illness but I think it can be broader than this).

I don't get it. I matched with someone over the weekend and we were having a good back and forth. We decided based on schedules to grab a bite to eat today. I had sent a message about watching the game last night and if she watched sports. I didn't hear back. Okay, no big deal. I made reservations for dinner and was going to send her the details, but we're not matched anymore. I'm like WTF happened? Oh well. I'll have a nice steak on my own.

For flakes, I only invite them to group hangouts. If they show up, cool, and if not, no loss. But generally I very rarely initiate 1on1 plans because I'm so scared of being ghosted

Ghosting became cool. How, I cannot say, but it was the end of integrity. Good luck out there!

They only care about themselves. They don’t give a shit about anyone else.

you need to provide better examples. we do not know how to respond

Ugh. I can't help but ask this question again. I mean, what makes people think being flaky and not keeping your own words acceptable? Culture? upbringing? education? mental issues? neurodiversity??