Can you name two such devices? Because the only thing I've ever found a Z80 in is an S1 MP3 player. I've found ARMs, 8051s, and weird Epson microcontrollers they seem to have never published the instruction set for, but never a Z80.
Not a hugely compelling argument for "the z80 is still a popular microcontroller", mind.
[0] https://en.wikipedia.org/wiki/Comparison_of_Texas_Instrument...
[1] https://www.amazon.co.uk/Texas-Instruments-TI-84-CE-T-Python...
[2] Mild cheating going on though because it runs the Python on an ARM copro.
TI uses them in one of their calculator lines. I've seen gobs of them as embedded controllers in invarious industrial systems (I have family in manufacturing). I understand a lot of coin-op games (think pinball or pachinko) use them. I've seen them in appliance controller boards (don't recall the brand).
So? That's like saying "there are no 8051s because Intel doesn't make them", even tho there are millions if not billions of clones made every year (since you'll undoubtedly say "well I haven't seen one", if your car tells you when your tire pressure is low, there's 4 8051s).
(It also matters whether you need things like an external crystal or three power rails.)
I mean, it's convenient when you can use an existing compiler, or at least can find documentation for the instruction set; and 8-bit instruction sets and address buses are constraining enough that they can really make it hard to do things. But these are not nearly as important as being able to get your code running on the chip at all.
So, no, instruction-set-compatible clones (like the ones I mentioned I found in https://news.ycombinator.com/item?id=43488079, the day before your comment) are not interchangeable with Intel 8051s in the context of improvising computational capabilities out of garbage. Pinouts matter. Programming waveforms matter.
With respect to the Z80-clone TI calculators, in https://news.ycombinator.com/item?id=43488344 Virgil explained that they can in fact run CollapseOS, but can't yet self-host because CollapseOS can't yet access their Flash. If you want to use them to control motors or solenoids or something, you still need some pinout information, which I'm not sure we have.