I know what you mean. With 9x19mm automatics available in so many sizes, pretty much all of them holding more ammo and offering quicker reloads than any 9mm revolver (and many of them lighter to boot), it is hard to see why anyone would want a 9mm revolver.
Yet if someone prefers revolvers over automatics, for whatever their purpose is, then a 9mm revolver can make sense. It uses ammunition with more kinetic energy than any 38 Special +P load, yet not as much flash, blast, and recoil as 357 Magnum, in a revolver that does not need to be much, if any, heavier or bulkier than a small-frame 38 Special. It's a niche role, I grant you, but it's one that seems to be big enough for several manufacturers to find worth serving.
Of course, they don't find it worth tooling up to make a revolver designed JUST for 9x19mm, which could have a shorter cylinder and frame than a 38 Special, but worth it enough to offer their 38/357-size guns with cylinders for 9mm. I guess that points to it not being all that big a niche either.
All this is just off the top of my head. I hope I have not missed something stupidly obvious.