Isn't it because the microwave is shielded against a very specific frequency, that is much lower than the 5G one? So that the 5G frequency penetrates the shielding without much trouble?
(I'm really not sure, that's my initial guess, but the questions are not rhetorical.)
Microwaves are at 2.4Ghz. Normal 5G ranges from 800Mhz till about 5Ghz. That is about the same ballpark, and Faraday cages are not incredibly frequency sensitive devices.
Isn't it because the microwave is shielded against a very specific frequency, that is much lower than the 5G one? So that the 5G frequency penetrates the shielding without much trouble?
(I'm really not sure, that's my initial guess, but the questions are not rhetorical.)