Why do low-quality bikes have horizontal dropouts, even though they aren't fixed speed? Though convenient vertical dropouts and standard derailleur mounting are appearing on low-end bicycles, a lot still use horizontal dropouts, without any fittings for trim to return the wheel axle to exactly the same position when it is removed. The hapless users of these bikes have to fiddle with the wheel after fixing a flat or whatever. Moreover, without those limiting screws, the axle can slip under load, jamming the wheel against a chainstay.

The only purpose for such a drop-out is that it provides a way to tension the chain on a fixed-speed bike. But these bikes will never be converted to fixed speed. So why do their frames have horizontal dropouts?

Is it because cheap frames cannot be manufactured accurately enough to have precisely aligned vertical dropouts, or is there some other reason?

Also, if I may stuff the question a little bit: why aren't the dropouts just used at their very end position? These bikes always have the axle positioned in some mid-position in the dropout, even though no such position is necessary for the sake of the chain, which is tensioned by a derailleur, and will likely work fine in any possible dropout position.   Ah yes: the derailleur hanger typically has a stabilizing screw that goes through the dropout also, rearward of the axle. But couldn't that be solved in some other way, though, like having that screw located forward of the axle?
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.