Sometimes, a game is just unlucky enough to get stuck with a programmer, art director or developer who just isn't particularly skilled. It's not that the game was intended to be bad, that's just the way it happened to turn out. Can't give a raccoon a paintbrush and expect "The Last Supper", after all.
Most of the time, though, it's to make some sweet moolah. Because it's an industry we hold dear, I don't think many of us like to think about it this way, but at the end of the day, game development companies are still companies. They understand that kids don't care about quality, and that said kids have parents with disposable income. Churning out a crappy game on the double to make some quick scratch simply makes more economic sense.
Let's say you're Sega, around the time the Iron Man movie came out. You can work very, very hard to make the movie based game an experience that every kind of gamer could enjoy and make a big sized bag of cash... Or you can throw together any old thing, slap the Iron Man logo on it, and make an equally large bag of cash.
