First, I'd question where Hasbro was getting all the money from. Say what you want about Studio Ghibli, but their movies aren't exactly cheap. For example, Spirited away had a budget of roughly 19 million dollars (and that seems to be one of the less expensive ones).
And while I'm not sure how much money Hasbro gets from MLP these days, I'll doubt it's enough that they're willing to throw away that amount of money on it.
Secondly, I'd question why Studio Ghibli would accept the offer. It's not like they gain anything out of it or have a reputation of adapting the work of other companies.