Of course, with phones it is more difficult to assemble stuff (everything is much smaller), but the whole concept of a standardized open platform is not there.
Somebody should define the "PC Phone", and declare it an open standard.
The platforms are well known, architectures are standardize, GPU drivers are often proprietary similar to the desktop world, but ultimately I can flash anything I want on my phone, I've run stock Android, several modified variants, Firefox OS, Ubuntu, Sailfish and others.
From a software perspective that's all not too hard to achieve, the hardware side of things does lack standardization but that's because as you say, it's not exactly something you can hand assemble.
But yeah, totally accessible.