Technical and design staff may make up a relatively small portion of the headcount for some of them, but the Windows-only enterprise is becoming increasingly less common. The primary reason being that the tools they need to support other operating systems are simply more capable and accessible than they have been in the past.
And as far as enterprise IT, there are a ton of Windows infrastructure folks, and again, there's little overlap between those and FAANG types. Plus infrastructure folks frequently hate Macs, for example (the stuff to manage fleets of Macs is vastly underpowered compared to stuff used to manage fleets of Windows PCs).
Plus even in those companies, the ERP & infra folks are maybe 1-5-10% of the workforce and they're generally in supporting roles (frequently they're temps or consultants), not in core business roles. Nobody listens to them :-))
And they generally want to get rid of them, they're seen as an expense.
I do agree that Windows-only places are becoming less common, but in big enterprises it's far from agreed that going outside the Microsoft ecosystem has a good ROI.
In any case, the reason enterprise adopts these technologies isn’t directly because of a technical ROI. It’s because giving people the tools that they want to use is generally seen as beneficial. Providing good UX to internal users is becoming just as trendy in enterprise as DevSecOps and Service Oriented Architecture is. There’s plenty of fairly obvious business reasons why enterprise would care about these things. One of the more important ones is that these organisations are competing FAANGs and startups to hire talent. If you walked around the bank office I was working in today I doubt you’d be able to tell it apart from a typical well-funded startup.
It’s true that large enterprise is slow and cumbersome. But they really don’t care too much if they’re buying you a MacBook, or a surface, or a Dell. Their main concern is whether they can properly manage all their devices, which isn’t too hard to do these days. Whether a large enterprise has MacBooks or not depends a lot more on whether or not they’ve figured out how to use JAMF than it does anything else.