Same reason "McDonalds hamburgers" aren't a market. They're just a branded product within a larger "smartphones" market.
> How do you define market, then, if that's not a market?
Same way the FTC does:
> all goods or services that buyers view as close substitutes
https://www.ftc.gov/advice-guidance/competition-guidance/gui...
An Android is a close substitute for an iPhone, just like a Ford is a close substitute for a Toyota. People switch between them all the time.
Competition is not a panacea.
In this case, there is already competition between distribution channels, the issue is predators/malafide behavior who will always be able to run circles around channels.
I suspect that the cost of review is not apparent - review at scale is going to be outsourcing. Thats a whole bunch of contracting, management, and sheer work that new distribution channels will not be able to afford.
Adding competition doesnt help. At some point, it simply creates more vectors of attack.
The situation you describe is better served by a policing force, that stops bad actors.
As it is, there is only one channel and no competition, so we don't know what the malware situation would be like in the absence of Apple's monopoly.