It's always going to be easier being employee #57 or #5,700 where you can have your job description and be given tasks to perform—maybe in a skillful way, but within a framework that is planned and managed by others. Every two weeks, money gets deposited in your bank account, even if you've been in a rut and have only been moderately productive. Want to go to a business event? Feel too sick to work? That's fine, you still get paid your full salary—no money comes out of your pocket. On top of that, you're guaranteed a paid vacation every year and probably have at least a basic benefits package.
It's a pretty good deal and one that most people are happy to take.
Forget all of that with a startup, at least at the beginning. But that's a big part of the appeal. You don't have a job description (or, if you do, it's pretty fuzzy) and you get to do things that no one in their right mind would hire you to do—developing a lot of new skills. You don't have to write a resume and go on job interviews to be asked silly questions hoping that someone will recognize your talents. You work with people you want to work with on the products of your choosing (preferably with a lot of market input) where you get to decide the strategy and how you'll execute. And you never have to stare at the clock and wish it was 5pm.
It's not for everyone, and if your startup grows into a bigger company, some of the freedom of the early days goes away, but for many people who have been there, there's no going back to being employee #57.