It's because of marketing. This is no regular transfer of a player, this is a transfer of one of the biggest marketing assets in the history of sports.
Look at our new logo for example. It's all about marketing in football nowadays.
This is not only Juve money, this will also be financed with 'Fiat' money. I'm almost sure about that.
Hell, I even think that this decision was made outside of Marotta and Paratici but by the Agnelli's.