The term startup here includes any private company under 500 employees. Companies like stripe and cloudflare won't count.
Add to that the hiring practises in startups are usually something like a quick chat, a tech test, and then an indepth interview rather than 6 rounds of interview and whiteboard tests, and just the process of getting a job in a startup is more appealling.
In my experience, larger organizations and government tend to have too many processes that distract from meaningful work and progress. People feel detached from the success of the company, I mean how much influence can you have as one person amoung thousands? It seems to lead to lower motivation and more tribalism.
Startups are personal, you know everyone, you can make an impact and when things go well you share in the success.
BTW it would be welcome to clarify of what to consider a startup since this word seems to be slightly overused.