"As easy as in desktop" not going to happen anytime soon. Phone "bootloader firmwares" lack full description of phone hardware in stardartized form like PC ACPI does, expecting customized OS kernel to know the phone it's running on. Phone devices lack capability to emulate something ancient-but-standard, expecting customized OS to include drivers for all of them. That's enough to make unified OS images impossible.

Fighting against well known hardware on PCs is one thing, doing the same against mobile manufacturers because they refuse to release any documentation is a nightmare. Linux will have a hard time becoming a reality in the mobile world because of hardware manufacturers hostility, not for any of its faults. Pine64 had to design the PinePhone platform from scratch for this exact reason, still they encountered so many blocks that when it finally became available it was already too old.

Nobody wants it outside of a tiny hobbyist segment.