Any question answered...

Is there a mechanical reason you can't get disc brakes with horizontal / semi-horizontal dropouts (not track ends)?

Rotor alignment? Surely that's a problem with track ends too and you get plenty of SS mtbs with those.

People don't like change. Also it's much easier to get a wheel in a vertical dropout when you have a derailleur. More people are making bikes with backward facing dropouts though.
There's also the issue of the brake force pushing the wheel out the back. In the usual position outside the rear triangle the caliper is above the axle. I don't know the exact terminology but if you jam on the back brake the wheel is going to be pulled backwards.
Not an issue if the caliper is placed inside the rear triangle but it does usually make for more manufacturing costs as the dropout/brake boss has to be machined out of a bigger piece of material.

Bolt through ftw.