But your phone's screen will (hopefully) never occupy 30 degrees of your vision like your computer's monitor probably does. It's not just the number of pixels, it's the number of letters your eyes can distinguish on the screen. And, while you may be able to attach a keyboard and monitor and interact with it like a PC, the normal situation will be that you only have the built-in hardware handy when you want to use it. Not to mention that your phone has a much higher chance of getting dropped and run over by a truck.
And, while phones are moving from "definite gadget" to "always gadget, sometimes also PC", there are plenty of ARM-based systems that are definitely not; you're not going to plug an external monitor and keyboard into your desktop computer's hard drive, even though it's also likely to have a more powerful processor than your first Linux PC before long. If anything, the popularity of non-PC gadgets which could sensibly run Linux (relative to PCs) is going to increase over time as more devices which were traditionally mechanical or hard-coded become smart. (I really want to run Linux on my washing machine. It's a great piece of equipment, but it doesn't always have a program for what I want.)