Part of that is seasonal. You're not going to get a stout in the summer, just like you're not going to get an Oktoberfest in the spring or a Shandy in the winter. Like you said about IPAs, there are so many variations, they've become a year round beer. Other year round beers are things like lagers and ales. Comparing a seasonal style to a year round beer isn't a fair comparison
If you can't find a stout on tap in the winter, you're going to the wrong breweries
Just because you drink them year round, doesnt mean most people do. They dont brew them in the summer because they cant sell them enough to make it profitable.
You definitely have a point there about only having ipas at a brewery. IPA is the easiest beer to make, and the easiest beer to hide any imperfections. 1 of our 2 main breweries in town basically only does IPAs. Took them like 5 years until they did something that actually was good and not an IPA