Why don't Travel Authorizations allow entry after travel?

A number of years ago we hired a consultant firm to implement travel (with a number of other modules). We had a lot of resistance to the idea that users would not be able to enter travel authorizations after travel. Rather than change the system in this respect we elected to replicate the TA approval process in our Expense Report workflow, making it more complicated, and leaving the TA function largely unused. I understand from TA not submitted before travel occurred. that the problems with approval after/during travel have been resolved, but we have kept the submission constraint. I am curious what the business reason behind this constraint is. From our business perspective we would like the ability to keep our more complicated workflow within the TA, and simplify the time to payment on the ER once TA approval is complete. This constraint is something we are considering programming around to enable more of our users to embrace the TA.