I don't get why major distros like Kubuntu don't just add a nice looking default theme.

("nice looking" by the standards of the vast majority of people, not some small group of hackers to whom Windows 95 was the pinnacle of design.)

I think Kubuntu looks very reasonable and switching KDE themes is as simple as it gets.

Where I think you are right is that it would be very feasible to create a few different builds of e.g. Kubuntu which come with different presets or make those available during installation, with easy switching in the life desktop environment. Maintaining each one should be quite simple, as it is just a few packages, with some configuration on top.