Because of all the memory safety techniques in the way. ASLR, other sandboxing in windows or kernel features, all make it extremely complex. You need to somehow get extremely low level access which ain’t easy to do.

Also do we know that the resulting electromagnetic wave will be powerful enough to be received in a reasonable distance? If it can only send data in a 3 feet radius it’s pretty useless.

Also a truly airgapped system would be in some sort of faraday cage which makes wireless signals themselves very hard to transfer, unless you somehow infiltrate a device, store collected data, and then exfiltrate it. So we are assuming that people are able to get past physical security in that aspect? Gets more and more ridiculous but it’s a very cool thought experiment