Reliability. This is basically the microchip version of Boeing's MCAS.
The circuit you describe is not only an analog circuit, but is in fact a noise amplifier. You're now shipping a chip containing a noise amplifier that drives the device-wide reset line.
What could go wrong?
The stuff you describe is very, very difficult to get right, and beast-mode insanely difficult to troubleshoot or even diagnose when it goes wrong.
It's also very sensitive to manufacturing variations. So if there is a problem with the circuit, it'll probably only affect a few batches. Which, Murphy's Law and all, will be the batches that wind up in the hands of your most important customers.
Stuff like this can bankrupt a chip company if you get it wrong, and there's no way to be sure you got it right. At most you put it in your super-high-end ultra-secure product line, so long as that line's sales are small enough that you can afford a recall.
It would seem very easy to put a pair of fets in such a way they detected sudden voltage changes (via their gate capacitance). That could then be used as an input to a circuit which ensured the chip is properly reset by asserting the reset line for at least 1 clock cycle.
This should probably be paired with brown-out detection, although that's power hungry, so I can see why people might not want it.
This wouldn't only have security benefits - lots of electronic designs might be accidentally glitching their microcontrollers due to poor design of other circuits, and having the chip reset in a predictable way is much better than undefined behaviour.