In that case you'd still need a way to convert stuffit files to 7zip - the reason this is needed is because it was common to distribute Mac software as StuffIt files back in the day, so there are large archives in the format that people want to read. There exist no 7zip archives of Mac System 6 software.

It's chicken-or-egg. No one (re-)distributes old Mac software in 7zip format because there's no 7zip port, and there's no 7zip port because all of the software is in stuffit format.