I would think that risk could be a part of it (or at least risk mitigation)...

If I were a photagrapher, I would likely want to charge more for a wedding then say the engagement pictures (or other portraits) which have a similiar amount of effort. If I have a problem with a portrait I might need to call them back in to reshoot. If theres a problem with the wedding pictures I'm stuck.

The workaround is better preparation, more people, more equipment, etc. All this costs money.