What's the same question you get asked over and over again?
"How much will it cost?"
If you tell your customer you work on an hourly rate and its $XXX - what's the next question you're asked?
"How long will it take?"
If you give a rough estimate here, the customer is now trying to add up what it's going to cost.... the problem being all the unknown variables like materials and unforeseen issues that may arise. The figure they've worked out in their head may be lot different to the end result, and now you're on the backfoot trying to explain why......