smaller pixels might make for smoother games
If the game is 3D rendered on the fly (so most PC games that aren't browser based, and a few that are) then the render overhead increases as you add pixels.
Instead of a fully lit dynamic detailed world with great view distances at 1920x1080 with anti-aliasing to minimise the grain on edges the same graphics card would have to lose the AA, the detail, the lighting, the view distances or the frame rates at 3440x1440.
That doesn't mean that you can't buy a bigger/faster graphics card, but it does mean it's now a lot more expensive.
More of a challenge is that most games wont support an ultra-wide format. They'll display in a 16:9, 16:10 or 4:3 ratio, so if your monitor can't deliver those natively you're now having the scale the output and it's not going to look quite as nice.