Why are grant indirect costs so high?
The indirect costs help to pay for all of the other resources you have access to which are not directly paid for by other costs. For instance, these indirect costs help to pay for administrative assistants, information technology, library subscriptions, utilities, and other costs.

They can also go to pay for the costs of administrators (including the fine folks in your university grants and contracts office who actually submit your NSF proposal).

Whether or not the rate is reasonable for your university is hard to judge, but a 50% overhead rate overall is not that excessive. Compare that to industry or the US national laboratories, where the overhead rate can exceed 200%, and things don't look so bad in comparison.