Because self-actualization is a human need and for a lot of people education is a means for that. Because some of us enjoy creating things that are of value to others. Because life is more than survival, basically.
Are we talking about UBI, or about education? Because if it's UBI, what I mentioned was just one of the mechanisms by which it could work. But I honestly don't know if it would. I suspect it would, but we can't know without more research and tests. Human behaviour is too complex to model a priori.