Why are all UX's so different?

Because the initial product launch required an astronomical effort to bring to market, and the final UI was the max they could do with what they could afford.

Product gains popularity.

"Don't kill the golden goose" (Wharton B-school MBA Rule #2)

The "evolutionary, not revolutionary" improvements come on a tangent that was set by desperation.

This is why #Menards and #Autozone cashiers are treated to a terminal emulator to a COBOL mainframe program.

And the term emulator's from Win 3.1.