Because people download viruses from the internet all the time? "Common sense antivirus" might work fine if you're technically inclined, but that's not the case for everyone.
The growing prevalence of so-called "supply-chain attacks" (a bad name because it implies a commercial relationship that doesn't usually exist) shows that "common sense antivirus" isn't working so well even among the technically inclined.
Because Google owns Youtube.
To protect the normies from harmful malware… not on their approved vendor list.
it's to protect shareholder value.
Because Google does no evol.
You could also ask why Android care about banning side loading to "prevent scams and spyware", and I honestly don't have an answer at all.