There are plenty of smartphone companies locking down their bootloaders, but there are others that will let you unlock your bootloader by just running the basic command.
A much bigger problem for running Linux on phones is that standard Linux runs like crap on phones. It doesn't have the mainline driver support amd64 computers have, and the battery life optimizations that make Android usable need to be reimplemented on top of Linux to get a day's worth of use out of your phone. Unfortunately, most Linux applications are written for desktops where they expect the CPU to be running all the time, the WiFi to be accessible whenever they want, and for sleep/suspend to be extremely incidental rather than every two minutes.
