Because I like them and want to drive one. Is that a good enough reason?
Olly lives in the US so would have seen this question in the same light as we'd have seen 'Hi, I'm coming to the UK on holiday and would really like to pay a huge pile of money to rent a vehicle like a Ford Transit LWB', hence his reply The ones I've seen on rental lots have had no rear deck cover either so there is very little luggage space as you can't use the rear deck for storage.
The reality is that as others have said you'll get a far better deal on a far better car by renting a fullsize SUV which will probably land you a Chevrolet Tahoe or a GMC Yukon. It'll be massive, it'll probably have black paint, it'll have a V8 under the bonnet and it'll be just as American as that F150.