Why don't airliners use light strips on physical controls (yoke/stick, throttle and trim) to indicate who has control - pilot or flight computer?
They do.
On the 737 there is a light in the button for each autopilot and next to the autothrottle switch. As well as annunciation in green text at the top-center of the Primary Flight Display (PFD).
I'll see if I can find, or take, a photo (that isn't copyrighted) to illustrate this.
The indications are there; the challenge can be getting thru the information overload so that the pilots realize what the aircraft is telling them.