People who purchase BMW, including the 3 series, are buying a sports sedan. Most of these other american cars thats listed are sports cars. If speed/performance is the only concern, of course BMW would not be the best choice. However, for most of us, speed is NOT the only thing we are seeking, we also look for other factors such as styling, comfortness, and luxury. BMW is definitely superior in these field, and thus the extra money we pay for them.
Let's use a food analogy. If your main concern is to get full, you can spend $20.00 at Wendy's buying up their 99cent menu and get extremley full on it. However, if what you want is to get full AND enjoy the taste while you eat the food, then you might want to spend $50 getting some prime ribs at Lawry's. Perhaps you won't be as full as if you ate at Wendy's, but probably you'll enjoy it a lot more.
So in conclusion... Why BMW? Because you can.