Why position z control is different form position XY control

I am studying position control of arducopter.


can anyone explain to me why acceleration is only used in Z direction however XY just velocity
can XY acceleration be used too?
much thanks

XY acceleration is directly proportional to lean angle so there is no need for a PID loop.

But then even vertical acceleration is proportional to throttle( thrust produced) - weight of drone. In that case why do we need a z PID for acceleration?