I'm assuming it's because 7-zip (1) has widespread use on other platforms (2) supports solid compression and (3) is efficient for randomly access. There is DAR

I'm assuming it's because 7-zip (1) has widespread use on other platforms (2) supports solid compression and (3) is efficient for randomly access. There is DAR which does (2) and (3) but not cross-platform and is already esoteric on *nix platforms.