This was explored theoretically a few times, but the problem is it's really hard for this not to become extremely anti-consumer.

iPhones dominate the cellphone market? You now have to figure out a way to split the manufacturing and selling of iPhones across multiple different entities. Consumers end up with a handful of worse products that all cost more.

When AT&T was split up, it was just turned into a bunch of smaller regional monopolies that were in a lot of ways worse. It also forced the closure of Bell Labs - one of the most important R&D powerhouses in the US. (Most of these regional monopolies would end up merging back into bigger monopolies over time anyway)

AT&T was much more heavily regulated to operate in the public interest in a way that iPhone manufacturers are not. It's not so clear-cut that the regional monopolies were worse because they were smaller, rather than because they were no longer as strictly regulated.