There are quite a few reasons.

Number one being, how do I truely know the fit I did is incorrect? They could be perfectly fine with the work I did that took me upwards of 3 hours but may still complain of pain and I would have to give their money back. Unfortunately it is not a tangible item that is returnable like a pair of cycling shoes that fell apart. I do however have what I believe to be some of the best follow up around. I will always do whatever possible to alleviate any fit issues my clients may have.

I deal with all types of riders; fat, thin, fit, not so fit, old, young. Some but not many come to me thinking a fit is magically going to bring them to the podium at their next race or jump their watts up 15%. There may be some complaints about comfort but are way overweight or have had 6 knee surgeries and a hip replacement. There are just some people that will never be comfortable no matter how well I tweak their bike to fit there biomechanics. Weight tends to contribute to many fit issues. Folks that carry extra weight put more pressure on contact points (e.g. saddle, bars, pedals/shoes). Heavy weight, previous surgeries, and injuries can limit flexibility and make it hard for a good fitter to get the bike to compliment the riders build and physical abilities.