Why Android can't be just mounted? I read a lot about rooting android, that it is risky, that I can mess all and get my device completely dead and nothing can be done about it. So my question -- why all these phones can't be just mounted? If I mess up my system with root, I always can boot with live-cd or write disk image directly to /dev/sda. After all, I can just mount broken system and repair it.

Why android to not support it? Is it some real technical restriction, related to flash memory or something, that differs between phone and PC/Notebook?
The only reason you need to root phones in the first place is that the manufacturer has tried to prevent software choice on the phone platform. The PC design is pretty unique in that it has had a choice of software platform for so long that it's an ingrained feature, although with Secure Boot on Windows 8 devices this is likely on the way out.