Re: Really, is it that hard?
You don't know the half of it. In the last few days I've resurrected an old phone, HTC One X, so that I can give it away or use it as a spare. I was somewhat overzealous at my first attempt at wiping it, in that I erased the operating system.
Fortunately, when it was my main phone, I'd installed a 3rd-party "recovery", TWRP. To do that, I'd had to apply to HTC for a code to unlock the bootloader. Why was the bootloader locked? Security? I don't know.
A usual manufacturer's Android has no root access and the system partition is mounted read-only. To replace a library, say, you'd need a special procedure more complicated than "sudo apt install libxyz". And there isn't one.
I actually installed the last available Cyanogenmod for it, an unofficial CM12, or Android 5 build. Potentially, I could now re-mount partitions read-write and update individual system files, although I'd probably break it.