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.
Based on my observations, I'd say it actually does alternate. My theory is that the motor speed is slow enough and consistent enough, as is the friction, that the motor consistently overshoots by half a pole when it stops. Thus, it moves in the opposite direction when turned back on. If it was moving faster, it might well be random. What's the chances of it displaying bifurcation then chaos? :-)
Given that the period of my oven is bang on 12 seconds, averaged over many revolutions, I think it's probably a synchronous motor.
Interesting observation on the random direction, I'd always wondered why that was.
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.
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.