It's a classic lease vs. buy argument. If you only need a resource for a day, then relatively speaking, it's cheaper. However, if you pay the rental fee every day for a year, it gets more and more expensive than an outright purchase. The comparison largely depends on the time scale you look at.

The main reasons I'm aware of that provisioning dedicated servers take longer is because you have to deal with data center logistics. Which rack does it go in? Which VLANs does it need to be in? What bandwidth is the customer paying for and how will we route the uplink? If the customer has other machines can we put these new ones in the same rack without overloading the PDU?

There's a lot of hidden work that goes into data center management.