Customers who book reservations and change their plans tie up inventory, and in some cases it is too late to be resold (although in other cases, UA is better off because it can be resold at a higher fare). One could argue that change fees could have a sliding scale based on when the change is made, but a counterargument is that is too complicated for customers.

In general, a change fee allows UA to offer competitive fares to those with firm travel plans, while generating additional revenue (to sustain those low fares) from customers who need the flexibility to change their plans.

Obviously WN sees things a bit differently, but some types of changes (such as switching return dates, or even same day standby) can be cheaper on UA than on WN.

Again these are my personal opinions/guesses and not the official policy/opinion of UA.