It's a cheap way to prevent abuse.

If you have no log in flow at all you'll be spammed. If you have your own sign in flow how do you detect and stop automated account creation and subsequent spam? Can you beat Google at stopping bot signups?

In the end it's a lightweight barrier to abuse. You can argue they should diversify and not just use Google but hey it's a small project. It's understandable. It's also free to sign up for Google for a real user.

Ever so slightly harder to create dozens of accounts to get free credits?

Presumably to harvest sign-in information