Because you always wanted to do academic research? Because you someday want to teach at the college level? Because you have no idea what you want to do and you want to keep your options open a while longer?
I left grad school to work in industry, but I’m not so naive as to think that everyone shares my preferences. I have friends who turned down Google offers c. 2001 to go to grad school, and they’ve never regretted that decision. (And I don’t regret the time I spent in grad school before leaving, either).
Personally, I'd want to get a PhD to be able to work in such places as Microsoft Research. I don't want to become a teacher/professor. Other than that, I really hate some perils of academic world - bureaucracy, nepotism/favoritism/elitism, ...
I left grad school to work in industry, but I’m not so naive as to think that everyone shares my preferences. I have friends who turned down Google offers c. 2001 to go to grad school, and they’ve never regretted that decision. (And I don’t regret the time I spent in grad school before leaving, either).