maybe partially because fps have been done to death to and there is not much you can do to seperate one shooter from another, tacking on some kind of rpgish system is a easy way to add something to a game, without straining your creative powers too much and spend more development time and money into creating some truly unique gameplay mechanic.
also since rpgs have become so bastardized you cannot really get gamers to agree on what constitutes a real rpg anymore, diablo is a game hailed by many but it a was a horrible rpg for me personally, i would point to this game as where a real schism began with rpgs, action rpgs, jrpg, wrpgs, every shooter to come out with some rpg stuff tacked into them, even sports games have some rpgish elements in them, sandbox vs linear rpgs games. but it used to be simple rpgs were rpgs there were no sub categories, you either were a rpg or you were not. fps were fps, rts were rts, sports games were sports games.
also since rpgs have become so bastardized you cannot really get gamers to agree on what constitutes a real rpg anymore, diablo is a game hailed by many but it a was a horrible rpg for me personally, i would point to this game as where a real schism began with rpgs, action rpgs, jrpg, wrpgs, every shooter to come out with some rpg stuff tacked into them, even sports games have some rpgish elements in them, sandbox vs linear rpgs games. but it used to be simple rpgs were rpgs there were no sub categories, you either were a rpg or you were not. fps were fps, rts were rts, sports games were sports games.