I'm not going to be intensely technical about this stuff. Because seriously anything above 30 fps in a game with maxed graphics is not noticeable, consciously. It becomes a problem when you drop below 24/5.
You also have many other factors to put in. Like the Hz rate of your monitor.
If you have a 60Hz monitor, you will not see more FPS than 60 even if you get like 400.
Let's put it this way, say you have a monitor capable of 4000 Hz. You play a game at 4000 FPS. Now after you play it at 30 FPS you will notice a huge difference in smoothness, a huge one. Sounds to me that you aren't paying close enough attention to it. Same goes for 120 FPS vs 30 FPS, any person can distinguish the difference, if you can't it actually sounds like a serious vision problem, not to be rude.
It's called personal experience. Anything above 30 is indescernable to me.
Again, it doesn't prove that the human eye has a limit of 30 FPS just because you can't tell the difference between FPS speeds yourself. Many people can easily distinguish 30 FPS from 60 FPS.
But yes I understand what you're trying to say, that 30 FPS is enough for gaming, which it is. But it's much smoother to get even higher FPS and it makes it more enjoyable.