Just à nitpick, consoles at that time didn't load levels to ram but rather the part of the level you're seeing. You could access rom as normal memory (rom banks considerations aside) so everything was already accessible (sometimes compressed)
You needed to store the state of the entire level, like what enemies are alive, what treasures have been collected, etc. and that turns into a non trivial amount of stuff to store in RAM (only about 2kb on the NES!) for large and complex levels. You can't store that stuff in ROM since it's changing as the player progresses. A PC at the time could guarantee 640k or more of RAM alone.
