The reason is convenience. There would be a lot more friction if they didn't do it like this for everything other than local development.

Docker also has more traps and not quite as obvious as this. For example, it can change the private IP block its using without telling you. I got hit by this once due to a clash with a private block I was using for some other purpose. There's a way to fix it in the config but it won't affect already created containers.

By the way. While we're here. A public service announcement. You probably do NOT need the userland-proxy and can disable it.

/etc/docker/daemon.json

{ "userland-proxy": false }