Because you're not a furniture mover. Hourly rates:

* Make it harder to negotiate good rates: https://news.ycombinator.com/item?id=3420782

* Are a pernicious drag on your client relationships: https://news.ycombinator.com/item?id=4103417

* Deprive you of opportunities to do lightweight things for free without setting weird expectations: https://news.ycombinator.com/item?id=4102639

* Obscure the actual value you're providing to clients, which isn't simply getting technical things done: https://news.ycombinator.com/item?id=4103207

* Are part of the difference between transactional contracting and consulting: https://news.ycombinator.com/item?id=2909923