Many customers will still choose airlines they like to fly. For instance, I filter Spirit Airlines flights, even if they are offering the lowest fare.

And also flight routes - I'd rather fly with a stopover in Singapore than a stopover in Dubai.

The incentive is to sell to people like me!

* In a second-tier city - no single airline has good options, but by comparing all of them I can usually find a only-mildly-lousy flight.

* Not especially price sensitive, I usually pay up to 2x the cheapest possible fare, to get a better schedule, seats, etc.