Because marketing told people it's the right thing to do. Other than that however, if You have midnight launch event it's like mini-con, with loads of freaky people in fancy costumes and often with some additional swag (t-shirts, posters, etc.)
I remember when WoW launched, it wasn't even midnight release,, but since MMOs weren't that popular back then They only had very limited amount of boxes sent out to shops, You had to sign up at least 8 hours before to even be able to buy it, but the shops also gave posters to first buyers. People love to get free stuff.