I can’t root my Android phone right now to install something like LineageOS, but yesterday, I sat down, installed ADB, and got rid of a mountain of unnecessary crap that comes packaged with Android and which is installed as a “system package” so you’re not allowed to uninstall it normally (e.g. Gmail).
Unlocking bootloader/flashing isn’t so easy for most people. I root every device I get, but for normal users this process isn’t easy. Also some manufacturers aren’t so friendly towards people unlocking bootloaders. I remember Sony lowering camera quality when you unlocked/installed Cyanogenmod back in Z2 times, or more recently Asus preventing you from unlocking completely.