I think it mostly comes from a place of laziness to be frank. A lot of people can't be bothered to learn the rules of the game they're currently playing, and mostly rely on learning the rules at the table from the GM or a more experienced player in the group, instead of on their own. So to these reading-averse motherfuckers the idea of a new game where nobody is more experienced to leech knowledge from probably seems pretty scary.
It absolutely is easier to just outright learn a new game then try to break 5e or Pathfinder apart until you can rearrange the pieces to poorly resemble the thing you actually want to play. However, to the brainlets that suffer from this particular hangup the idea of putting the upfront effort to learn a new game is more intimidating than putting in an assload more overall work to rebuild 5e into a completely different game.
Edit: I should also say, being averse to learning a new game seems to mostly be a player issue, so if a GM has to rebuild 5e into a sci-fi space opera because their group wants to play a sci-fi game but refuses to learn a new system, all that effort is now on the GM to rebuild the game and the lazy players can coast by doing nothing.