They're common in catering, for easy cleaning, but require more sophisticated design of the waveguides.

I think Panasonic still make some. The term they use if you're web searching is "flatbed".

My parents have an ancient one, which they inherieted from a relative decades ago...

It has a button to cycle through the power levels (the typical couple seconds on, couple seconds off duty cycle power levels), and four buttons to set the time: one under each digit (adds one to the digit and overflows to zero), the start button, something for temperature cooking if you have the probe (they don't). Actual buttons, because touch buttons hadn't been invented yet. A total of 8 buttons, plus the door open.

Anyway, no turntable there.

Same here, but unlike my rotating one, that one had classic coldspots and hotspots microwaves sometimes had.

But I sure loved the fine grained time and power control. Figure out the setting put once and then it is unattended perfection every time.

Yeah, my parents have an upside down ceramic pie dish permanently in the microwave which raises the food to be heated up into what seems to be a more favorable area for heating. Before we got that in there, you'd have to do a lot of short bursts and mix / rotate, which is tedious especially when there was about two seconds of off time when starting...

Before they were integrated into the microwave, you could buy third-party turntables.

They were like a child's toy; you would spin it a dozen times counterclockwise, and then it would slowly rotate clockwise for the next several minutes. Obviously, it couldn't be battery-powered.

Thinking about it now, it must have had some kind of plastic spring mechanism. Wish I had busted it open and looked.

they still make them, although this one seems to be out of stock on Amazon.

https://www.amazon.com/Nordic-Ware-Microwave-Micro-Go-Round-...

I think that’s how most “commercial” microwaves work; the food stays put and the element moves. It’s much easier to clean and it (usually) heats the food more evenly.

There are a few now, though they're still a minority, we have a Sharp SM327FHS which has the same arrangement - flat bed with a hidden rotating emitter.

Seems good so far.

(It does not have the physical dial though, it has buttons like most other microwaves)