Every enterprise I have worked at had both linux and mac available to developers. Is this not your experience? You seem to present as truth the opposite of what I have lived.
"Developers" are a miniscule share of "enterprise". We are talking about the billion computers on corporate office workers' desks, not about the maybe-a-million computers of web/network/systems developers.
Counter-anecdote: every place I worked at including 4-5 major companies in various industries had compulsory windows desktops for everyone. The few of us that also had a Linux (or AIX/HP-UX/...) machines that we did our work on, SSHd into those machines.
I never worked on an enterprise with GNU/Linux worstations on their IT policy, on some companies IT tolerated rebel developers provided they took care 100% of integration issues with the company network and official tools.
Macs are usually only available to upper management, unless required for iOS development, in which case they are part of a pool shared among project teams.
Not at all. Linux can sometimes be made available on servers if necessary, but if you want Linux on your desktop you have to use a VM (ideally without telling IT). Never worked at a large place where Macs where readily available. Macs used to be more popular ~10 years ago or so with the marketing and communications people doing a lot of DTP and design work, but even that seems to have greatly decreased.
That's because there has never been a <$999 MacBook. If you purely start to look at the Ultrabook segment then MacBooks have an absolutely staggering market share.
To be fair in my limited experience it is true. I live and work and Switzerland, the majority of customers i faced with, i.e. medium to bigger companies, universities and gov agencies all used Macs. I travelled as Digitalnomad, and went to plenty meetups around Startups, Entrepreneurship and other not 100% tech things. Clear mac dominance as well.
However i work with Rails, Python and fancy things. I know if i would look into .NET/suit wearing side of things, things would look very different.