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.

Because Google owns Youtube.

To protect the normies from harmful malware… not on their approved vendor list.

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.