I worked at a startup of a semiconductor fab early in my life, which was not quite the same as what you mean but I think the reason still applies. I wanted to see how these messes get made. After working in old organizations with lots of legacy problems, I wanted to see first-hand what the process was that led to these situations. It did.

Having worked as a contract programmer at software startups, I can say that the main reason would be, to have more power to direct how the software develops, instead of just having to accept so much of it as a problem on top of which too much as been built, and which cannot be fixed.

However, I worked at a software startup only as a paid-by-the-hour contractor, because frankly the attitude towards early employees time is appalling, and I have seen this in how many, many other people got treated at other startups. There really isn't enough of a potential payoff for early employees to justify the hours that they are made to work, and in many cases it is done mostly because it is so easy to demand that and no justification is required. Founders are by nature egomaniacs with ruthless focus on their business, and that can turn even very good people into the sort of person you don't want to work for (unless you're charging by the hour).