I believe that's why they're calling it "a phone", or "a tablet". The computer they actually sell has plenty of shells available, and lets you tinker with whatever you like.
A phone is not simply a computer, it's a regulated piece of hardware that must comply with local laws and regulations regarding radio transmissions and other stuff. You can't just peek and poke around anywhere you like in the system.
Besides that, it must be able to talk to carefully tuned 3G/4G/5G cell towers, which sounds easy in theory, but it's not. When I made mobile phones 20 years ago, we had people driving around all countries where we sold it, with a test setup where the phone connected to every cell tower it could "see", and recorded logs and GPS coordinates, and that work (and that of countless others) is partially what became the beginning of A-GPS (https://en.wikipedia.org/wiki/Assisted_GNSS), which allows you to triangulate your phones location purely from the cell towers it can see.
Of course that's not how it works today, as most carriers these days register their cell towers in a central database with GPS coordinates, so A-GPS these days is simply a database dump (and a whole lot of math).
As a "fun" anecdote, when I wrote software for mobile phones, it was the only place I've ever worked that had a bug category for "potential harm to user". I'm certain companies working in Medicare and other critical industries also has that, but it was the first and only time I ever saw it.
