Of course you can try to negotiate for more. Enough people want to work at a startup however that if you ask too much, you'll get passed on, and they'll just hire the next candidate who doesn't act so entitled to a meaningful piece of the founders' pie. It's exactly the same phenomenon that lets the big game studios chew up and spit out engineers: there's always someone else willing to take the job for less, or even just put up with the status quo.

That's a massive part of the problem, and telling people to negotiate better doesn't address it — or even acknowledge it.