It's primarily an RPG. Almost every game is some kind of hybrid nowadays. At some point in the development cycle, every developer has to make a decision on what their focus for the game is going to be and in ME2's case, I think it's clearly the RPG genre. With that comes the need for concessions to other genres and their style of play.
Sure, WE might like a game like that but for the 98% of gamers who don't aggressively play 30+ games a year and have thousands of hours logged into both shooters and action games, the game would probably become absurdly difficult to play and almost impossible to control. I had casual gaming friends who had problems with the original ME on the normal difficulty setting. Developers have to keep those people in mind when they drop $20m or more on making a game.
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