We have a culture of service as a legacy from un-fine old steel such as Schwinn drainpipe sadly not yet rusted away.
Yet modern components operate close to the limit of tolerances.
A customer tightening a loose bolt or an inexperienced technician can ruin a frame or component.
Centering a disc caliper can offer fewer frustrations than centering a single-pivot side-pull. Yet the young technician struggling with the single-pivot will not likely harm the part nor put the customer at risk.
Steel bolts into alloy threads require judgment and a more skilled technician. Do we charge more to center a disc caliper? Or do we wait for the store down the street to change their prices or go out of business.