Yes, everyone should respond. From the hiring manager side, it's sometimes hard (especially when people argue with you) but I still think it's worth doing.
I'm sorry you're having a hard time. I'm sure it's very demoralizing to feel like you're applying into a void.
I'm not sure why you think companies have an obligation to talk to every candidate that applies.
It's pretty common to get hundreds of applications for any open role now, and the most visible companies get thousands of applications. A majority of those applications can be disqualified from the resume alone.
It would be a massive waste of everyone's time to talk to every applicant.
YC invests on a SAFE, the terms are public.[0]
For most companies, pre-seed SAFEs don't end up much above common.
[0] https://www.ycombinator.com/deal