Because they something to lock you in to Ubuntu. They want Ubuntu to be the only thing that uses snaps. They want to get snaps to be an Ubuntu exclusive feature, and once they can start convincing some random closed source devs to ship in only the snap format they have a hook to keep you on Ubuntu. And they want those random random closed source devs to be focused on more of the corporate world so they can sell some support licenses.

Download the netinst image. You get the option of installing a de or none entirely.

proprietary file extensions

What file extension is that?