Because there's no such thing as a free lunch, running your own datacenter (or managing datacenter managed services) is work, and the clouds are better at providing exactly the compute you need when you need it than buying a ton of excess hardware you might or might not use (or can't scale quickly enough if you _do_ use it all)

A datacenter makes sense if your usage profile is steady state and hardly ever changes, or if growth rate is predictable and capacity can be procured in advance. Any other use case is better suited for the cloud, IMO.