I think theyre still popular on Usenet.

IDK all the history but rar has built in ability to create recovery data / parity volumes.

Parity data is like additional data that can help reconstruct any degradation.

That said, usually a standalone parity generator is used which can work with other types of archives, but rar is what everyone uses so why change.

Compression algos are ineffective on encoded / compressed media anyway.

It used to be important on Usenet, and maybe still is, because if a drive starts to fail somewhere and contains errors those errors can be reproduced across the network. Not sure if thats still a thing or why but certainly 10 years ago it was.

The summary to this rambling comment is: some communities still like rar because its what they've always used and there's no benefit to adopting 7z.