If you buy a hot dog at a ball game, there’s not much surprise in the outcome (hopefully). You pay your money, you get your food, all done.
When you buy a professional service, on the other hand, it’s more complicated. With things like executive coaching, or financial advice, or management consulting, there are many more unknowns.
Your clients are making a best guess, but they don’t really know what it’s going to be like to work with you.