Because the point of starting startups isn't to create a successful business. It's to trick VCs into giving you money to burn! /s
Taking investment is, at best, a necessary evil for startups. It's not a victory. It's never a victory. It's a compromise. It's "I just sold a hundred million dollars for a million dollars" - with the awareness that if you hadn't sold it, you wouldn't get the other hundreds of millions either.