Because privatized carriers are low-margin, high-cost, and with an uncertain future. Let’s take two different paths as an example of why it’s a bad idea.
PATH 1: Privatized Carriers are the Forever Norm
In this scenario, private carriers (cellular, wireline, satellite, fiber, etc) don’t get nationalized or regulated (so now, but forever). The market forces there are all about consolidation since these are low-margin businesses, and fewer competitors means higher prices, more revenues, and more customers. If Apple entered this arena, they’d be exposing themselves to the predatory maneuvers of companies like BT or AT&T, who generally enjoy screwing their competitors over and locking customers in. Now these telecoms would have justification for blocking Apple devices or functionality (like some did to FaceTime), and deliberately choosing radios the iPhone doesn’t support so as to force Apple to spend more money on expansion of their network or adopting new radios for said networks. It generally exposes Apple to a world of pain and misery for nothing in terms of growth, and hinders their technological progress by forcing them to invest exponentially more money to support new standards or products across their network.
PATH 2: Carriers are Nationalized, with Telecom treated as a Utility
This is a more likely outcome given the importance of the internet to the functioning of a society. As a general rule, if your country NEEDS something in order to succeed, that thing is cheaper and more scalable if you own it yourself - provided you have a functioning government apparatus that invests in its modernization and maintenance (so, not America). In this case, Apple would’ve started their own carrier only to see control taken from them for the public good, wasting money they could’ve invested in other R&D or product lines. That’s likely the long game they foresaw back in the 2000s, and is likely why they never started their own MVNO or carrier.
Ultimately, Apple benefits more from building products on internationally standardized communications systems than building said systems themselves. It keeps their costs low, lets others absorb R&D and upkeep costs, while also driving the market and infrastructure adoption through new products themselves.
Infrastructure is rarely directly profitable to private enterprise, and when it is, it’s often taken away once it becomes necessary for the survival of a country.