Usually the "major" differences between each of those yearly releases are technical "gameplay" mechanics, things that actually effect how the game is played I guess. I asked a friend of mine something similar to this just a couple weeks ago when he bought Madden, and he told me that various things get changed between releases that the "average gamer" wouldn't know at it's face value. Apparently, player rankings actually matter "more" in Madden 17 vs 16, some physics or something got changed with tackle dynamics, new plays are created etc etc. Stuff like that I guess is updated per game to give a "more realistic" representation of the game or whatever.
You also have to remember this yearly sports release thing isn't anything new, this has been going on since the days of the Genesis. I think that's the main reason it's continued as it has, it's just "always been that way" and people expect it to happen.
That said, I mostly agree it's a pretty shit move to continually re-release a game like that for full price when it could be easily done with updates, or even yearly "DLC" I guess if they still wanted to make money. Or hell, even release a new version every 3-4 years when devs can further "refine" the engine/graphics or whatever.
But people still buy it, so who cares really? It's not like the devs that make these game are some kind of "big loss" to the gaming community, they don't make anything other than sports titles AFAIK so I'd imagine any other game genre they'd try to develop wouldn't end up being that great (not that the devs are bad at making things, I just imagine they're too used to working with sports titles to create a successful non-sports genre game)