The reason why is how many cores the game runs on. If you do an all core overclock to 4ghz then each core is running at that speed. But if you are running it at stock, then the single core speed is running a 4ghz. So if the game likes single core performance (which most do), then you are not seeing any benefit. Only at the times when more than a couple of cores are used.
In 2018, overclocking really only helps in heavily threaded applications. Basically, workstation task and gaming is not heavily threaded. Not yet. Now that 6 and 8 core CPUs are mainstream, in the coming years we will see games start to take advantage of those extra threads.