Because gzip and bz2 exists. 7z is almost always a plugin or addon, or extra application. While the first two work out of the box pretty much everywhere. It also depends on frequency of access, frequency of addendum, size, type of data, etc. If you have an archive that you have to add new files frequently, 7z is gonna start grating on you with the compression times. But it is Ok if you are going to extract very frequently from an archive that will never change. While gz and bz2 are overall the “good enough at every use case” format.
7z can be at least decompressed in macOS & FreeBSD out of the box.
On windows tar.bz/gz/xz unpacks to tar and then to actual files. As tar is a separate archive format
Windows having tar.gz support is great.
I have scripts for generating log bundles on user computers and sending to a share. tar.gz is great for compressing ~2.5GB text to send over VPN, and then I can open the .tar.gz direct from the network drive with minimal additional delay opening a 500MB text file inside.
