Googles/Apples argument would have been much stronger if their stores managed to not allow scams/malware/bad apps to their store but this is not the case. They want to have the full control without having the full responsibility. It's just powergrab.
Scam apps are rife in the iOS App Store. But what they can’t do easily install viruses that affect anything out of its sandbox, keyloggers, etc
I agree let's have sandboxed app instalations on platforms. Flatpak is already going this way. But it looks like big players Microsoft,Apple and Google are gatekeeping app sandboxing behind their stores instead of allowing people/devs to use sandboxing directly.
If users without that level of technical skill are pressured into making those decisions, that's because they're being mistreated.