60fps is set, because of a most common screen frequency. Most popular LCD are 60Hz, which gives you 60 "refreshes" per second.
If you (in theory) set your game to 120 fps, it will not be displayed anyway on most of people's screens, that are 60Hz.
So... basically... whats the point?
Unless you make an app strictly destinated for (more specialistic) 120Hz screens.
I don't think you could make more then 60fps with Construct 2 - in the documentation it says one "tick" is set to 1/60 sec.