You learn so much
The number one reason to work at any startup is simply how much you learn. Startups are high feedback environments, and high feedback environments are high learning environments.
I have grown more in a few weeks at a fast moving startup than in almost a year at a university.
Startups can also be places where people freely upskill each other and share knowledge openly with a spirit of uplifting the whole team. In larger orgs this becomes political and knowledge tends to be hoarded, job titles and remits protected and time for education is scarce.
Given that startups often struggle to hire the most qualified talent, you'll also often be given training and formal education to upskill you. Building talent is incredibly important for smaller teams.
There's also the fact that in a small company you necessarily have much more visibility and access to all the other functions, including leadership. Collaboration tends to be common and sometimes even required. With many gaps in the org chart as the team works to get to 1.0, there are often many opportunities to put on a new hat and learn on the job, mucking in and getting your hands dirty in an area you aren't familiar with. Devs doing support, designers doing marketing, sales doing fundraising etc.