Why can't I search on fees?

A lot of people would love to be able to search on fees. "Show me a payment gateway with less than 3% transaction charges". The problem is, there are so many ways of calculating fees that it's really not possible to compare. Some charge a percentage, some a percentage plus a fixed amount per transaction, and some differing amounts depending on the transactions you do. They also vary based on monthly fees, other features added (such as fraud detection) and so on. As a result being able to search by fees is impossible in practice.