Every large company has certain dependencies (compute, networking, and storage in this case, although you could argue for ex. food service, HR, maybe financial tooling), and those dependencies entail cost and risk, with more risk from using external vendors. So eventually, big enough companies build their own. They build their own datacenters, write their own provisioning tools, design their own network setups, flesh out their own HR departments. And then they look at it and say, "hey, if we can safely sell this, we can recoup the costs of building it, and maybe turn a profit." And if their system is general enough, and they can deal with security well enough, then usually they do just that. Compute is easy to rent out. Networks require some work for security but it's quite doable. HR tends to be too specialized and/or sensitive to resell. (But not always, and sometimes you do see companies reselling employee portals and whatnot.)