This is dangerously untrue for many categories of app. Google plays a game of whack-a-mole trying to stop the worst malware but it’s far from perfect and once you’re in more dubious categories like adware they pretty don’t bother.
That’s the problem here: it’d be nice to be able to install Netflix from Netflix.com but making it that easy means that millions of people will get something with bundled adware, spyware, etc. because they didn’t realize what they were installing - or because it wasn’t installed by them but their controlling spouse, kid who likes games, etc. The long-term answer for this will involve better OS sandboxes but as we’ve seen that’s a tough thing to get right and will inevitably limit what people can do or be abused (e.g. taking away direct access to other applications means attackers now try to convince people to enable assistive features).
Now, maybe that freedom is worth the risk. Personally I think it is but I also know that statistically the rate of people I know/support getting their credit cards stolen, being used in a botnet, or having their PC become unusable dropped to zero when those people switched to iOS or ChromeOS so I certainly can’t say there isn’t a solid argument that the general public cannot use general purpose computers safely.