People did starting as early as 2004. The kits were roughly $50-75, including required PCB until as mentioned, the scalpers took over. At that point, most of the several older collectors that made them said "$!@# it" and stopped making them the last being Terry around 2007, if I remember correctly after Stern SM.

Disdain was the primary reason, or people simply wanting a free handout for nothing.

Remaining collectors who want one, continue to build them from spare parts since that time.
It seems to be a "mystery" to some today, but the details are still all over the internet.
Whitestar and SAM compatible.
SPIKE is not easily possible without additional game code modification.

I built one for my MET PM as well, works fine, no issues, but this is SAM. I also installed one in my LOTR, SM, WPT, TSPP, and a few other games in the past, as these are all WhiteStar. They all have the coil programmed for use with the transistor, SPIKE games due not directly have the feature, and I doubt that the new SW will have it either in either in the programming or parts. The voltage issue and driver are not the hard part, the programming is the hard part. An owner has to "trick" the computer into sending a signal to a new coil via code rewrite, and this is not the same as say replacing songs on a game.

The same can be said for other game kits as well made in the past such as the original Mark Davidson Dalek "wobble head" kit and the CTFBL LCD kit due to poor quality attempts at copycatting by others, and the original makers basically felt snubbed because new owners did not know the difference between each of the kits anyway, but were being asked for help on kits they did not make.

Periodically a collector will build a "one off" for another friend, but they will not advertise anything as they get mobbed with requests or undulated with installation problem questions.