Assuming modern microwaves have a microchip on which computation can be done, and an electric motor of controllable speed, couldn't the microwave calculate the speed required for the specified time to end at the right angle, with no delay?
Appliance manufacturers go for the cheapest parts possible that still perform reliably under the warranty period. The motor driving that turntable is not a precision stepper or brushless motor with variable microprocessor speed control. It's a cheap A/C motor (perhaps with a drive belt for torque) that's switched to line current by a 30-cent mechanical relay. You can tell it's A/C when the turntable starts in a random direction each time it's energized.
There is no need to put an expensive polarity-reversal circuit (with mechanical or electronic memory) in the assembly just to alternate the turntable direction. The cooking effect is the same.
Given that the period of my oven is bang on 12 seconds, averaged over many revolutions, I think it's probably a synchronous motor.
The other thing to remember is that in the 1990s a microwave oven cost several hundred dollars at least. Now you can pick one up for fifty bucks, possibly forty. Losing extraneous features like "boomerang turntable" is presumably part of how the manufacturers managed to respond to the downwards price pressure.