they can clear active memory, clutter, if you will, but it cant clear certain aspects of the games cache especially due to the games open world concept. larger world = more objects to load = closer to the memory cap. older games = more memory to load due to progress = closer to memory cap. higher definition = far more memory used = closer to memory cap.
dont get me wrong, the game does clear clutter, but its direct coding is designed for a non modded game on medium graphical power, on a very obsolete System specs. and was designed for THAT in mind first, then adapted for the more powerful, and modable PC. sadly, this means that the games going to crash cause some memory just cant be erased no matter how many times the game tries to clear the cache, as it still can not clear everything. And you will always notice more crashing, with the more mods you put in, or how much progress you have made, as both add that extra strain to the system.
I use over 100 mods in my skyrim game, it looks utterly fantastic.
I crash on average about once every 1 - 2 hours, with some places, especially near water, I tend to crash more frequently. (I crash a lot near solitude) but Im running a top end computer that runs every game on extreme graphical settings while still getting 60 frames easy.