Most organizations use both on-prem data centers and cloud-based IaaS services, often employing multiple IaaS platforms.
For some, this multicloud reality has come about as part of a steady, one-way migration to the cloud, and they may have intentionally kept their cloud networks distinct as part of that goal. Others may have a business strategy for keeping them distinct, such as providing services for a stand-alone division or a particular geography.
As a consequence, they are almost certainly already tying their on-premises and cloud infrastructure networks together in some way or are about to be.