Because...
1) Its an option, it hasn't happened yet.
2) All clubs have an "option to buy", we just have a price already agreed.
3) Coutinho is here because of Gerrard. After this season, Gerrard has a year and a half left on his contract, coinciding with Klopp. Do you see Coutinho committing to a three or four year contract at Villa if Gerrard is possibly / likely here only 18 months?
Its a wonderful loan signing and I can't wait to see him play. However, the "biggest signing in Villa's history" talk is hyperbole gone crazy.