The cost of a round trip ticket can be as much as several times the value of a first class upgrade. Those who take frequent plane trips may find it a better option to spend two or three flights enjoying the amenities of first class seating than getting a single free coach flight after spending a small fortune on other flights just to earn it, not to mention tacking all those extra fees onto the “free” flight.
Frequent fliers must often fly many times in order to earn a ticket, which minimizes the financial benefit of getting one free flight. First class seating will allow a flier to be more comfortable, which may be a more worthwhile benefit than the alternative. Upgrading also usually does not come with additional taxes and surcharges.