Because it's a rule as far as I know (20 field players + 3 goalies) and we have more attacking option as any other team out there.
Loew has an offensive dilema. Goetze was weak (why where we sending crosses into him?) but is in there to get the best out of Ozil but that comes at the expense of Mueller. Mueller plays best if the team has a big target man (Gomez) but that comes at the expense of Ozil.
Try listening to a Boateng interview. Guy is actually pretty smart, but his mic skills are beyond awful
boring game. There are no pacey players for Germany on the wing. Why couldn't it have been Gotze being injured instead of Reus?