The vast majority of malware isn't delivered via play store because of the existing measures and protections they have. Same reason you see very little app-store-based malware on iOS. DISCLAIMER: YES MALWARE EXISTS ON APPLE HARDWARE PLEASE DON'T SHOUT AT ME. Talking specifically about anything installed via first party stores on both platforms.

Their main issue is this: dumb people install apks from spurious website and infect their phones. The least controllable and most pervasive factor here is the intelligence and knowledge of the user which cannot be controlled for by Google. So by eliminating the ability to exploit this entirely, it will eliminate that specific vector.

It's a sledgehammer solution that naturally comes with many downsides like disrupting intelligent and knowledgeable users that just want to hack around with FOSS and such.

Google is relying on It being too expensive for malware creators to have to guide each individual user through adb installation and usage process just to get access to their phone. Most scammers only do that level of interaction to extract actual cash/gift cards from the target.

I am personally and directly affected by their decision in many negative ways, but I'm not so dense as to not understand why they're doing it.

/corpodronespeak

EDIT: bots help Xitter maintain inflated usage figures which justify people's jobs, share prices, etc. Bots are a feature, not a bug.