I think it stems from seeing leveling as two different things. Some see leveling up as a reward, and others treat it more as a pacing mechanic.
For me, leveling up is a way to pace the game. It goes from simpler to more complex. You start out fighting weaker enemies like goblins or orcs, but move up to more epic battles with fiends and ancient dragons. You have more things, more hp, more gold, more spells, more options.
So, leveling isn't about rewarding someone for doing whatever. It's about evolving the game over time. And, if the game is evolving so that it's about being bigger heroes doing bigger things, it makes absolutely no sense to say one player character is big and impressive and another isn't.
To this end, I also 1) keep everyone at the same level and 2) call for leveling up when I feel like it as the DM based on how I want the game to move. Both of these things detract from level as any kind of reward. It's hard to feel like you earned a level, I would suppose, when everyone gets it on DM whim instead of tracking it based off of some, in theory, neutral manner such as XP for monsters defeated. In this kind of case, level being a pacing mechanic is painfully clear and it is weird to even think about it as a reward. When that happens, having PCs at different levels would feel like a penalty or at least arbitrary.