Because it's a lot simpler and avoids the issue of dealing with printer drivers on all your machines.

I could see the argument that it's more air gapped this way. Without having physical access to the Pi (or at least SSH access), it'd be hard to get any network connection through USB.

But personally, I just blocked outgoing traffic from the printer.

Because the built-in networking stack on printers is garbage and having to install drivers on every client sucks.