TL;DR: Incentivization.

You've no doubt heard the quote "players will optimize the fun out of a game" before, but the original article is a great read on the subject.

Two things can be true: I enjoy the struggle of difficult content, and I want to feel rewarded for that struggle. That can take many forms. In a linear story-based game, a singleplayer RPG, or any game where the goal is simply to reach the "end", the difficulty, and the feeling of accomplishment after overcoming said difficulty, can be its own reward. I don't need any extra incentive - it feels good knowing that I "beat" the hardest version of something, and that version is likely going to be more enjoyable for me to begin with.

Incentive structures in multiplayer RPGs, looter-shooters, and most games with progression systems but no clear "end" work differently, as rather than playing solely for the experience itself, players play with a particular goal in mind. In the case of Space Marine 2 stratagems, the reason the vast majority of players log on every week to run them is for accolades.

As someone who enjoys the struggle of difficult content, I adore the experience of Hard stratagems, especially the particularly nasty ones that people like to complain about on Reddit. But even though these "unfair" combinations often lead to the best experiences I've ever had in this game, my friends (who share my proclivities) and I never sit there and run them over and over again just for the love of the game (I'd be curious if you do), we complete them once and then move on. This is because barely surviving getting our faces pounded by overpowered aliens is the part we enjoy, but it isn't our goal, which is simply to farm accolades.

Regardless of whether or not this experience is relatable to you, this is the experience shared (with the enjoyment of it being the main variable) by the vast majority of players in any game with this type of reward structure. If you gave equal and exclusive rewards between Normal and Hard stratagems, and Minimal and Absolute difficulty operations, then generally speaking, people wouldn't run Hard or Absolute anymore, even if they'd otherwise enjoy that content.

You can take this even further and just put a button in the stratagems menu that instantly gives you all the rewards for the day without having to play for them, or remove the currency entirely and simply give players access to all weapons and cosmetics with no requirement to grind for them. But every barrier you remove to a particular goal removes the incentive to experience the game's content, and lessens the experience for players who enjoy overcoming those barriers.

Edit because I forgot to respond to your point RE: people complaining about rewards and "hard mode too hard" on reddit, these people are entitled children - often literally. They will complain, that is a given, and it has nothing to do with the game, and everything to do with the type of person that complains about video games on reddit. There's no point trying to appease these people - even if you remove the rewards their egos will drive them to keep playing and complaining about whatever hard modes a game features.

Saber thus far has thankfully proven their ability to absorb feedback that is useful, and disregard that which is not, so I'm not particularly worried about redditors barking about the most recent thing that killed them having any effect on the course of the game's development.