If you want an honest answer, at least my opinion on it, it's because the entire concept of match making is frankly flawed.
For starters it's depending on a machine algorithm which always ends so well. algorithms are horribly unreliable in consistency. That's been proven time and again over the years. It is amusingly ironic considering it's the one thing a machine should do better.
Second how do you judge whose at what level?
Do you base it on W/L? Not only can that just be luck or based more on your friend list, but that essentially punishes people for doing well. Such a design literally makes the game harder for winning which isn't fun. Progress should result in sign of improvement which should translate into things getting easier in some way, but this kind of match making consistently ups the difficulty well beyond the speed at which you progress in games like this.
You can actually see it in action in this game which makes me pretty confienent in saying W/L ratios are at least part of the match making. If you win say ten or so matches in a row suddenly you'll start seeing players who are much, much more powerful then you gear wise. Then when you inevitably lose those matches you'll be back to the power level you were dealing with before until the cycle repeats itself. Once you hit the gear ceiling (rank 7+) this stops being a problem as at that point enemies can only be your equal or beneath you, but until then it can be infuriating.
Do you base it on K/D? Again could be luck. Right time and right place can get you a lot of kills from pure happenstance. Could also be good tactics which would result again in punishing players that play well because eventually they'll be up against enemies who out gear them so heavily they can't make up the difference.
Do you combine the two? Still walk away with that whole problem of the game being harder the better you do. Also now you've introduced an easy exploit into the system for people who don't care about stats: That is loading a match and doing nothing. Just accepting the loss, repeating that a few dozen or even hundred times, purely to throw the algorithm off and give them easier fights.
Do you base it on hangar power? Less flawed, but now the whales will complain because suddenly spending 300$ on the game doesn't let them win easier it just puts them up against people who not only have equal gear, but likely have much more experience and thus better tactics. They don't want that even if they say otherwise. What they want is to spit 300$ at the game and then stomp peoples faces in. Not getting that instant gratification is pretty likely to make them quit and games like this can't afford that.
Honestly I have never, in 30 years of gaming, encountered a match making system I consider good. The best games are the ones that either have pure random match making (that is to say, none at all) or very, very loose match making. Something like "well we'll start looking at your hanger power/KD/WL/Whatever, but if we don't find anyone in five seconds we'll just expand the search out to higher/lower powers until we get a full match."
To be perfectly clear I fully believe no match making is the best set up. Truly random is the most "fair" approach. No match making actaully rewards skill and investment of time (or money) more simply because the better/stronger you are the less likely you will be to encounter people you can't beat. If you're in the upper 10% of players that means roughly 90% of the time in a truly random system you'll be winning on at least a personal level.
Another way of getting around some of these problems is resetting the entire system every couple of weeks. If everyone is dragged back to the lowest rank at, lets say, the end of every month then the more skilled players get to feel good kicking the shit out of everybody for a few days before things stabilize again. Afterwards the weaker players won't be at the absolute peak of their statistical performance any more so they won't be stuck in that never ending cycle of "win three matches, then get curbstomped without a hope of victory, repeat." It also boots people who get by more on persistence or friends back to their proper rank so they don't end up getting constantly outclassed. During such resets most people don't care about running into a powerhouse player as much because the reaction will be "well they'll be long past me soon anyways."
People say what they want is a fair match. The vast majority of the time I've found this to be a lie. People want to win. If they win a fair match even better, but ultimately the win is more important then the fairness. They want to come out on top and who they're up against doesn't matter much. Honestly, if this game offered full co-op bot matches (5 players vs say 5 -10 bots) I would play little besides that and I doubt I'd be alone in that. I've seen it in every single "competitive game" that offers PvAI modes. Most people just want to shoot guns at mech, win the round, and move on to the next. Challenge isn't really what the vast majority of players are in for.