In the most basic sense - they should be concerned about malicious code because they're busy advertising and distributing those apps for a cut of the profit.

If Google were to say "No more checking for vulnerabilities in FDroid... (or insert other)" I would agree with your take - that seems like common sense. Not their store, not their problem. Same for side-loaded apps.

But that's not what's happening. They're busy selling those malicious/vulnerable apps for a cut of the profit.

Now - Google can be a responsible party here without having this program (there are plenty of valid discussions around whether this was really an effective way to combat malware on their store) - but to recap...

The store doesn't get to absolve themselves of responsibility for the things it's selling.

"It's the store's responsibility not to sell me malicious/defective products". If they can't do that... maybe they shouldn't be allowed to operate that store anymore.

What a ridiculous idea. You think your mother should or is capable of auditing the Facebook app before installing it?

Google promises that apps in the app store are safe.

It's your wall socket, it's your car, it's your organs that need surgery, therefore it's your responsibility to make sure it's electrically safe, mechanically safe, that you do your own surgery correctly, that your boat, etc. Yeah, nah.