Probably cause that's what it is and it doesn't fit your argument. It is not just a beat'em up. Beat 'em up is a larger umbrella term, Bayonetta, Devil May Cry, Ninja Gaiden, Metal Gear Rising are all a specific type of game that aren't remotely similar in execution to the Arkham series. It's a narrative you're pushing that doesn't fit at all
Combat doesnt exist in a bubble. Arkham has shallower combat and that is okay because it's not solely dependent on it with the games being built for a wider audience. Your criticism is very vague and your demands for more are even vaguer honestly. You keep asking for more depth, why don't you elaborate on what you want? People probably aren't largely demanding for a better combat system because they are already satisfied with how the combat has been implemented, and critical reception seems to back up Rockseady here.
Critical reception, with all due respect, means nothing. If we're going to take critical reception as some concrete fact, then why bother? let us all post MC scores and get done with it.
As I said, it's the same genre, the prevelant excuse for a game not having a deep combat system is exactly what you just said, it's the same excuse for the weak combat of RPGs like The Witcher 3, ppl are like "well, it's an RPG, what did you expect?" , I emphasis again, genre has nothing to do with how deep your combat system and mechanics are. It's like saying a movie which has a great story doesn't necessarily needs a compelling cinematography, you should work on delivering the best aspects of a product, just because you nail one aspect you shouldn't gloss over the rest.
Now about Batman Arkham series, as I previously stated, it nails the feel factor, it's fun and satysfying as hell, it's punchy, it's crunchy, and it's fast, which leads to a satisfying combat scenario, and I love this, but when it comes to depth, there are more elements in play, one is Skill Ceiling, meaning that there should be a difference between a rookie and a hardcore player, when skill ceiling is there, the players can express themselves, it's called Player Expression, the best instance for this is Bayonetta, just check out Youtube and see how hardcore players express themselves in that game and remind yourself of how you were doing, you can easily see the difference, now why this matters? this adds to the
replayability value, the more time you put into it the more you salvage from it, now how Batman is not as deep as it should be? 1. The timing window is so generous and you get a warning! hence you can EASILY counter the attacks, this hurts the skill ceiling, 2. plenty of gadgets in combat often become redundant because they really don't add much, this hurts player expression, this is why you don't see an interesting freestyle footage from Batman, you as the player can not be expressive. 3. Regardless of challenge scenario, you can always go with a similar tactic and nail it, the game doesn't encourage you to try other tactics.
I can go deeper but I wanna refer you to a perfect video on this for further and better explanation:
https://www.youtube.com/watch?v=VxRxh8Ka5H8 (Watch it from the beginning, tried to set the timer but don't know how
)
The reason why Batman shines is because ppl categorize it as Action Adventure so it gets to be compared with Assassin's Creed, Sleeping Dogs, Shadow of Mordor, etc, which all have mediocre af combat, but when Japan beat-em ups come into play, Batman pales in comparison.
And I wanna say this again, Batman Arkham series is one of the best games I've ever played (It's in my top 10), I love it, I had a shit ton of fun with it, me being critical here doesn't mean that I don't enjoy it, I'm just saying it could've been better, it could've had more depth, that's it.