When you’re playing online, each mission has a weapon limit. So if you’re playing mission 50 on Easy level, you can probably only bring up to level 7 or 8 weapons, something like that. It basically forces you to always be at an even matchup with the level, which is a bit unfair given how insanely punishing this game is.
It does force you to diversify a bit though, as you’d need to constantly be updating or changing your weapon lineup depending on which mission/difficulty you were playing. I prefer being able to find a good farming level at a higher level, grab weapons, then use those to clear out some tougher earlier missions on a higher difficulty, etc.