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.
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.
For archiving/backupping *NIX files, tar.whatever still wins as it preserves permissions while 7z, zip and rar don't
Oh, and while 7z is FOSS and supported out of the box on most Linux desktop OSes and on macOS, Windows users will complain they need to install stuff to open your zip. Somehow, tar.gz is supported out of the box on Linux, macOS, and yes Windows 10 and 11!