That's a good point. A bit part of the success in cloning is from convincing the plant it's vital to grow roots as fast as possible. The other part, of course, is providing it with anything it needs to do this.
Roots provide water and nutrients to the plant, so to need more roots the plant has to have not enough of one or both. Excess transpiration through the leaves creates water-debt that forces it to grow roots in response.
A high humidity environment helps prevent the plant from dehydrating and dying too fast, but if you give it too much moisture it can be lazy and decide roots aren't quite as vital.
You don't really have to worry about nutrients generally if your cutting is big enough, it can use stored energy to grow for many days.
The one thing I give them to help things along is hormones. The natural plant hormones used to grow roots, provided in abundance, help out a lot.