The main reason why companies had so many individual servers was that performance and capability could be compromised unless applications ran on separate ones, it also made maintenance easier as well as the fact that servers were relatively cheap compared to monolithic mainframes.
As companies have grown either organically or through mergers and acquisitions so have their server estates. This server sprawl has led to inefficiencies in the use of data center resources so many companies have aimed to mitigate it through virtualization.
There is also the issue of cost. The standard specification of a server purchased to host virtual machines (VMs) is usually much higher than a standalone server. Hosting multiple virtualized servers requires more CPUs and significantly more memory both of which contribute to higher power requirements and heat output.