The difference in compression isn't that big. Just ran a test on some random 1GB plain-text file I had lying around. Both on the highest compression level at a dictionary size that uses about 5.5GB of RAM to compress (since that's what 128MB dictionary size uses in 7zip, the Winrar equivalent are 916MB, more than what's useful for this file). The result was 7zip compressing the file down to 225MB in 9 minutes and WinRAR compressing it to 239MB in 2 minutes. That's a 6% difference, at a considerable speed cost.

Not a scientific benchmark, but I think it underlines general point. If I want the best results I use a .tar.xz at insane compression levels, or more commonly a .tar.zstd if I want good decompression speed. The usecase for 7zip and WinRar is convenience, ease of use and windows-native file handling instead of the unix-focused .tar format. WinRar wins out on all three of those.

7zip's gui is a worse clone of WinRar's, archive creation has a fraction of the features, windows-specific file handling is an afterthought at best (winrar has handling for the archive flag, alternate data streams, file security, hard links, etc). And most important of all rar is built as an archive format. You get built-in recovery records, and hashes are stored as blake2 hashes instead of the frankly insufficient crc32 hashes 7zip uses.

I'd give 7zip points if it had a better (== more familiar) CLI, but they made the bizarre decision to copy winrar's cli too and make even worse documentation for it. The only things it has going for it are a linux UI and the open-source license