(I always tend to think of over/unders being more expensive, but that may not be for the plain jane models).
And just where are the "plain jane" over/unders? Not saying there aren't ANY but they've never been common, or popular.
O/U's are literally, more expensive to make than a SxS of equal quality. The design means the action has to be "deeper" because the barrels have to open farther than a SxS which adds complexity and cost.
So, even a "budget" O/U can't quite compete cost wise against a SxS which does the same job. SO traditionally, O/U designs were mostly made as higher end sporting guns, not field grade working guns.
Coach guns come from the era of riding shotgun on a stagecoach. And back in those days, O/U's were not in popular use and pump guns didn't exist.
So, O/U vs SxS, the O/U is more expensive than the budget SxS and is slightly more awkward to reload, which of course is no nevermind when youre shooting birds, but a bit more important when you're trying to stop hostiles.
I don't play Cowboy games, but I do have a coach gun, a low cost Chinese made one (have tested works fine) for my wife to use in an emergency. She's not a "gun person" but does know what to do, and the coach gun I have has double triggers and double hammers, it doesn't get more basic than that.
When it comes to defensive shotguns, the pump gun rules, (with a few semis) and the double SxS is right behind that, the O/U just isn't in that market, never was, and because of that, it never will be.