I think the main reason third-party ROMs aren’t more popular is that Google and certain app developers fuck with people who use them. The article addresses the difficulties later on, but comes up short in my view on just how much of a hassle it is for someone who isn’t a tech enthusiast who wants, for example to keep an older phone up to date for security reasons.
I think the main motivation for Google is limiting user control over the experience. More user control leads to unprofitable behaviors like blocking ads and tracking, which is also the motivation for recent changes to the Chrome web browser that make content blocking extensions less effective. In all cases, companies that try to take away user control claim the motivation is security, usually for the benefit of the user.
That’s not quite true, though in that case it’s about the service provider being unable to verify that the user isn’t running a operating system configured or modified to work against the interests of the service provider.