At least in the case of the U.S., almost all of our wind tunnels were build decades ago before the technology was available to do this. I used to work at the USAF base that has most of the US wind tunnels. Most of them are quite old. One of them was actually taken from NAZI Germany after the war, though I think that one is mothballed.
As far as the reasons they aren't widely used, these come to mind:
It's a lot more complicated to design and build. In engineering, you normally choose the less complicated solution if it's good enough. Apparently the existing ones are good enough because we've been using them quite effectively for several decades.
I'm sure it would require an enormous magnetic field, as those models can be quite large (several meters) and, thus, quite heavy. Given all of the very sensitive sensors in the tunnels, it would likely be a pain to keep the field from interfering with the rest of the electronics. This is especially true in the engine testing tunnels, since the engines themselves are full of electronics nowadays. Almost everything in modern jet engines is electronically controlled.