On my car, if I manually override the cruise control (i.e. put my foot on the throttle) the cruise control disengages until I manually reengage it.
This works fine in a car, but isn't really applicable in an aircraft. Having AP and AT that automatically disconnect on manual input might sound like a good idea but there is another factor maybe you don't consider. Suppose that someone accidentally nudges the yoke or a throttle and doesn't notice it? I can recall at least one CFIT incident where this happened and the aircraft flew gracefully into the ground while the crew were unaware that it was no longer on AP.
If you think about it the way it actually works is very sensible: if you make a manual control input it lets you do it, assuming there is some urgent need, but if you don't follow up by explicitly disengaging the automatics it assumes that you don't intend to continue manual control and gets back to automatic control.
Over at Boeing and Airbus there are thousands of experts who work full time on developing these systems. Some of the things they do might appear daft to outsiders, but often there is a reason for things working the way they do.