Why is it so hard though? I guess the problem is always power management, and I guess writing drivers for the appropriate hardware? I still fail to understand

Why is it so hard though? I guess the problem is always power management, and I guess writing drivers for the appropriate hardware? I still fail to understand why there isn't more open source projects like this using android, since android is open source. I mean using linux on a desktop is easily feasible, but "owning" a tablet or a phone seems like it's so difficult.