Why do low-quality bikes have horizontal dropouts, even though they aren't fixed speed?
I feel certain that the reason comes down to cost.  Quality frame builders surely hold the seat tube, chain stays, seat stays, and the dropouts in a fixture when being welded together.  But surely accurate alignment costs money: more expensive machinery, fancier robots, and more skilled workers.  And of course frames that fail the dropout alignment test must be rejected or reworked.  Having horizontal dropouts eliminates these costs.

After having spent years maintaining a series of cheap department-store bicycles (BSOs) with chromed-steel rims and bolt-on axles, I was thrilled the first time I took the rear wheel off my first quality bicycle.  A quick-release skewer!  Dropouts with exact alignment!  A wheel that stays true, even!  With the BSOs, half the work in any maintenance procedure that involved removing the wheel was getting the rear wheel alignment correct after reinstallation so that the brake shoes didn't drag on the rim.