Yes, you have to plan, yes you have long lead times to get new gear, DC space, etc. But once it's running, failure rates are pretty low. At least in the single digit thousands and servers.
I've had to deal with much more frequent and odd types of failures on cloud infra than with on-prem.
Supply chain management and coordination is, IMHO, the most difficult part of it and is often overlooked in these discussions. Herding vendors to deliver hardware on your timeline from overseas supply chains doesn't always turn out the way you want, even with the best planning.
Isn't that the target audience for this sort of change?
I mean, do you see teams of networking , siteops, high availability, and security engineers hanging around doing nothing and just waiting for a company to decide to go in-house?
No, because those teams do not exist in free-range. That's something a company needs to build and train and experiment from the start until they are able to learn all the lessons.
If a company was going to run their own data centers, they would presumably hire someone that knows what they are doing to lead the effort instead of trying to do it by trial and error.