Not everyone has the money to pay market salaries, especially early-stage startups running on meager angel funds. Add to that investors who still actually think you should bootstrap yourself on ramen before trying to get any investment to properly pay your team members and employees. There's simply no way a talent-hungry startup could possibly pay anything close to what Google, Facebook, et al. offer. People who choose to work for the startup instead are doing it because they identify strongly with the problem or market, want to have a voice, or want equity. Those people also tend to be highly talented folks who value their opportunities to apply their talents much more than the monetary compensation.

Freedom, ego, and voice have always been a bigger attractant for talent than salary, assuming basic life necessities are met. I've turned down multiple $300K+ job offers just because they were too boring, and simple brain rot would drain me of all the abilities that I have invested time learning and accumulating over the years.