I've noticed this as well; of course it's particularly a thing round these parts because the city is full of pretentious bars that, as you say, can get a good mark-up on Fever Tree mixers, usually to go with the designer gins, flavoured with "Botanicals" that people tend to favour round here.
Mind you, I often drink Scotch and dry ginger if I can get it, and you don't find many pubs that sell diet dry ginger of any description nowadays. If we're going somewhere like a festival, I take my own supplies with me, and just buy neat Scotch from the bar, but in pubs I have to default to red wine (not necessarily a bad thing), or Scotch and soda. I have raised the subject in a couple of pubs but invariably get the reply that "we don't get much demand for it, mate". Grrrrrr...................