Because instead of having a million projects, you'd have 3. The power of defaults and opt-out is pretty well-understood to the point I'd call it non-controversial.

So what? That is a typical use case for an opt-in platform with possible good outcomes. If you do it opt-out, it's unsolicited, unwelcomed and just badly setup. This discussion and the one on Github itself is a perfect example why.

You can't blame someone else for not having a responsible (e.g. opt-in) growth engine...

They would have projects from people who are interested in their services, and interested in having them accept money on the projects behalf. If that number is 3, so be it.

Which is why you have to be way more aggressive in just about any endeavor.