What makes these platforms attractive to the end user is that they're the only things available for sale down at the phone store. And what makes the decision between the two available choices is what your friends have, or what you already have, or what's on sale, or about a trillion other things that are more likely to be "top of mind" for the average buyer.
I bet not one user in 1000 gives any thought to "app curation" before they choose a phone.
I very much appreciate how freely I can install apps from the app store. I hear about, I install it, I try it out, no worries. Whereas without curation I'd spend twenty minutes making sure it was mentioned by multiple sites or people I trust and doing a set of web searches to check for reports of malicious behavior, and I'd still worry about it, especially about updates. Putting out a good, well-behaved app and then putting out a malicious update that is required for the app to work with the latest OS update would be a common malware vector.
I'd probably uninstall half my apps every major OS update. How often do I use this app? Once a month? Is it worth doing a few quick web searches to see if the latest update is malicious? Nah, just delete it. I can do without.
Once or twice I've had to install a major OS update just a few days after it was released, which would mean no time for other people to discover malicious app updates. What would I do then? Just roll the dice? What if the initial update is fine, but then a week later it's replaced with a malicious one, to catch people who put off updating?
What if someone else did all of that privacy research and monitoring for you, only it was a set of volunteers, and they did it for free? This is what F-droid does except they verify that apps on their store are clean at the source code level, compile the source code, and then publish it in their app store on behalf of the authors. When I'm looking for an app in the F-droid repo, I only need to wonder, "Is it any good?", because it is at least safe. They also warn you if the app does anything at all you might object to. For an example, check out the page for Firefox below, which at least gives you an idea of the kind of information available in the app.
At times, Apple has removed apps capriciously out of self-interest and done things that aren't good from a privacy perspective. I trust F-droid to be a more equal an honest arbiter more than Apple since they don't have and reason to do something I wouldn't like. Basically, I'm saying that (in addition to scummy app stores) there would likely also be some app stores that do a better job than what Apple does at protecting you from the bad actors you just mentioned.
I completely agree with you on the value of curation.
But why not have a setting deep in the menu to allow sideloading unapproved apps? If Joe Block is able to opt in to the uncurated minefield, does that harm you or take away from you ability to enjoy curation?
I bet not one user in 1000 gives any thought to "app curation" before they choose a phone.