Why are all my games capped at 105 FPS?

Start by removing the NVIDIA global FPS limit entirely as a test, rather than setting it to 180. Also check the per-game driver profiles and each game's own frame limiter. A game profile can have different settings from the global ones.

If the NVIDIA app has optimized the games, inspect their settings too. Turning off optimization won't necessarily undo values it already changed.

Try the main monitor on its own, with the 60 Hz display disconnected. It's a quick way to see whether the mixed refresh rates are involved, not proof that the second monitor is the problem. You could also set both displays to 60 Hz temporarily and compare.

Check for a system-wide power or performance mode outside the NVIDIA settings. Armoury Crate, for example, can have Silent or Eco profiles. If it's installed, try Performance mode and retest. Also look for any other frame-limiting software or overlays running across games.

Test with G-Sync or FreeSync and variable refresh rate disabled temporarily, then compare the result. Those settings are worth isolating, though a flat 105 FPS doesn't by itself identify which setting is responsible.

Low overall CPU and GPU usage isn't conclusive either. A limit can leave the hardware waiting, but overall CPU usage can also hide a busy individual core.