I think you forget that many of these instruments have _antennas_. While they are very well insulated against unwanted interference, it's not like it's feasible

I think you forget that many of these instruments have _antennas_. While they are very well insulated against unwanted interference, it's not like it's feasible to protect against _any_ interference. After all, the pilots are talking on an unencrypted air band with the tower, and the same is the case with DME, VOR, ILS and GPS.