And if comms fail?
Comms are there to facilitate communication, not be the only source of communication. Comms and buzzer flags all help to introduce redundancy into the system.
I agree with others on here that the flag is also a communication tool for everybody else.
Also, FIFA have always tried to make the amateur and pro game as constant as possible (well, so they say). I know there's a number of things introduced which go against that now - VAR, AAR, etc - but those are things brought in at the top level. Makes no sense to flip that and take something away at the top level, to have ARs have flags at grassroots but not at the higher level