Because everyone else pirated to great effect.

They offer 3 options for doing so, in fact, in direct response to this controversy.

I can limit interactions to non-new users.

I can limit interactions to users that have contributed before.

I can limit interactions to users that I have manually whitelisted in my repo.

Unfortunately, only one of these can be active at a time, but the options do exist now.

Go to your repo -> Settings -> Moderation Settings -> Interaction Limits

Limiting interactions is not the same as disabling pull requests wholesale (you can disable issues completely), they are temporary in nature (you can only disable them for 6 months at a time), and in addition I believe my comment was posted before they announced the new feature -- and this is a feature people have been requesting for a decade now.

Thankfully, they've gone and made it opt-in from now on.