MajicMan: The old games, lots of people played.
Excitebike and Punch Out too, even if you knew nothing about Supercross or boxing, but who buys and plays modern sports sim games like FIFA, Madden, MLB The Show these days if they aren't fans?
Who else is going to drop $60 on a football game every year that also requires a $50 online play pass, go through career mode, create a character.
Who has knowledge of Cover 2 and 86 defenses? Play Action, West Coast, Run and Shoot and K-Gun offenses? The difference between a Nickel and Dime package. Do you like your line or linebackers more? Then are you playing a 3:4 or 4:3 among the seven up front? Are you playing man coverage or zone?
It's one of the reasons I really like Super Mega Baseball. The game is easy to pick up, play and have fun, but has a ton of depth and difficulty for fans of baseball. On easy modes you can just toss and swing and win, but on harder difficulties you need to pay attention to pitch selection, location, fatigue, how you pitched the batter last time, are your pitching patterns predictable and the Mojo meter.
That's the difference. All of the games I mentioned plus Excitebike and Punch-Out aren't overly complicated if you just want a game to play for 20 minutes or so. The rest of the games are more like simulators. There's nothing wrong with more realism, if you're into it, but people are more into mindless fun when they play. Not many at all care about something so much that they'll take the time to really learn about it, but that's true for everything. :)