>why not just use a system-wide package manager?

I mean lets not pretend that's a good choice for either devs or users either. From the user side, you're a the mercy of how fast your distro decides to package new versions. For most things that's fine, but for your main product you might want something newer. Of course devs can setup their own repos but then the devs have to do additional packaging, host infra, etc.

From the dev side, getting packages accepted into various distros can be a pain in the ass. Just look at the recent blowup around the Python Cryptography package when they decided to add Rust and Gentoo's complaints.