That is by design.

With an actual computer, you have a lot more options in terms of how you can handle the content. They don't own your PC (yet) but they do own the mobile market and the operating system entirely.

You don't have any form of root/admin access on a mobile device of any type nor any other device (tv box, game console, tablet, etc). For the handful that have jailbroken or rooted their phones, many apps don't work (by design of course). You can do it, but you'll break a ton of stuff in the process. That's enough to keep causals from doing it and leaves the tiny group of hardcore dorks that are willing to live with the complexities that are required.

I'm going to put on my tinfoil hat and say this is what keeps any real attempt at a Linux phone in the gutter. If people had a choice for their mobile operating system and the freedom to do what they want with it, the big tech companies would shit a brick. They've already removed the ability to block ads on mobile devices for the vast majority, they're finally getting what they wanted (save the handful of Firefox users left).

Linux phones won't be a thing until there is hardware for them. With apple/google phones, the manufacturers will not release the necessary software so that is needed for a 3rd party OS. Google and Apple will make sure that anyone who makes hardware for them are legally tied to keeping that software tightly closed. Even if someone did manage to reverse engineer it, it would be a herculean effort and would break as soon as a new release is out (or if one of the manufacturers "accidentally" released some exploit code for the reverse engineered drivers).

It's an ugly world these greedy fucks have made for us.

Yeah last I've seen even smartphones and tablets are decreasing quickly relative to smart TVs.