But I mean, why not take the 100s of thousands of man-hours that went into making Android this very Linux-Kernel based mobile OS and build on top?

Will you be happy with xeyes and a terminal? Like, even a technically superior solution is completely useless without an ecosystem to make use of it, and desktop GTK/qt apps won't work nicely on mobile without actual porting. Let alone a technically significantly inferior one that is a misfit in this shape for mobile hardware.