Why does drive thru give me my total after I drive to the pick up window and not then and there at the menu screen when ordering using

Because if the person filling the order told people the price, many would try to pay right then and there. Causing a lot of confusion. The 'filler' has been specifically forbidden to tell people the total.

Among other reasons, they don't want you trying to count coins while your car is moving toward their building.

It's usually because the person isn't at the register when they're taking your order. They have a lot of other job duties and move around the restaurant. Sometimes they're making food, completing another transaction, washing tables, and so on. There's also usually more than one person with a headset, but they wear them for different reasons. One may be assigned to be the order-taker, while the other assigned to collect cash/ hand out orders. Sometimes a manager will wear a headset just to listen in, while other times someone will be assigned to be an expediter for the drive through. They may pack bags, make drinks, or do other things to keep the line moving. Sometimes, you may wind up with one of these folks who is not the assigned order-taker taking your order in an effort to expedite things or the order-taker may respond to you while they're in a different area of the building and one of those folks will hear it and plug it into the computer to help them. Ergo, the order-taker may not even know your order has been listed in the computer until they get back to the register. 

If it bothers you and you like to dig for exact change, you can always ask for the total, but it'll likely throw a wrench in their otherwise well-oiled machine and slow down the processing of your order.