How many companies actually hit this stage? I can only think of a few, and usually it's because they have very specific hardware requirements (e.g. Dropbox's whole business is file storage, or if you're doing something that requires tons of GPUs).
In practical terms you should expect to never hit it. The point where such scaling (also the main value add of aws/azure) really matters and you start looking at an entire DC to lease, you've arrived in the realm of speculative fiction. You should not plan to get there, just as you should not plan to get a winning lottery ticket.
Also, it’s about priorities and goals of the company. Security and control is the main reasons I see companies migrate to data centers. Generally things like GitHub Enterprise are being used.