Think about this...
"Memory" is the only thing in the entire computer that "is (almost) as fast as the CPU." It's the one-and-only system resource that the CPU doesn't have to wait for (much...).
Therefore, the system is designed to "fill it up." Once something has been loaded into memory, even if that "something" is no longer being used, the system won't go out of its way to replace it or remove it. It'll allocate unused memory instead (if it's available), in hope that "the previously-loaded something" will again be needed very soon ... a quite-reasonable assumption.
"Laziness" is a virtue among operating systems.
Today, most computers have a plentiful supply of RAM. So, "laziness" is richly rewarded with excellent perceived performance. The computer will not make any attempt whatsoever to be anything but "lazy" until, and unless, actual memory-pressure materializes.
Then, it will begin "an organized retreat" from its lazy position... withdrawing first one use of the "extra" memory, then another, then another.
All modern operating systems follow this very successful strategy. This is why, if you want to make a computer "more zippy," you should always add memory (buy as much as it will hold...) before you contemplate buying a faster CPU.