It's happening now. Some are using the car's existing sensor-cams (Tesla) and others offer a 'white-label' rebranded cam (IIRC some Citroen models), but it only seem popular with Tesla owners because it is so easily implemented there. Neither approach gets you the kind of dashcam coverage, functionality, and image quality that most seek from a dashcam. And TBH the average dashcam owner doesn't check their cam or footage much if at all unless they need it for a crash they're involved with which doesn't happen very much, so there's little risk to a car dealer in warranty work.
There are very reliable dashcams made now, but same as always not every cam fits this category and there's still no cam made that doesn't benefit from having a new card when the old one gets worn out. The cams with reliability problems are usually newer models which aren't yet completely sorted out, or are certain brands who are known for unreliability. And generally, this kind of optional equipment doesn't carry the same warranty as the car itself does, so most cams will get the dealer past the warranty period on them.
Something to remember is that people rarely mention anything unless they are having problems- that's why most of the content on this forum and others seems like always and only bad news

For each person here reporting problems there are big numbers of people not having problems. And too the kind of people you find here are what the computer world used to call "power-users" who want to use every function to it's fullest which is beyond what the average user does, so we're naturally going to find more issues. Most people are unaware of what dashcams are capable of doing today, and most of those people buy on price which wil;l always be cheaper as an aftermarket buy that one bought from a car dealer. I think we will see more of the Tesla approach as more and more cars are getting optical sensors for the purpose of various driving aids, but until there is more motivation for them like car insurance discounts, they're not going to be hugely popular whether optional equipment or aftermarket
Phil