"combined with the fact that it would be simply stupid for a company who makes their own OS to run anything but that OS"
Why would you assume that? There are a ton of things that linux does better than OS X - and it would be extremely stupid for any company regardless of size to not use the right tool for the job. For example, even IBM uses, sells, and supports Linux instead of AIX or OS/360 on their line of servers and mainframes. I think that your assumption is just really old fashioned.
Internally Apple does use Linux, just as Microsoft uses a blend of OS's - supporting Linux on Azure, for example. I read that they actually use Linux as a host for their Hadoop service on Azure.
At it's core OS X is Unix. In what way would Linux be a better choice? I am not saying that Linux is a worse choice, but for a company that writes an OS as one of it's core businesses, it only makes sense to run that OS in as many places as possible. For one, by running OS X as a server OS they would necessarily spend more time on development and improvement of the OS core. This would pay off in the long run by further improving stability and reliability of OS X.
I am not arguing that OS X is the perfect solution in most circumstances, but it can be a good solution in many situations, especially if you are Apple, and have the full source and the capability to adopt the OS as necessary.
Microsoft, especially nowadays, tries to be very cross compatible, so it's not surprising that Azure supports Linux apps and guests. But Azure RUNS on Windows Server 2008, not Linux, not Unix.
Because it isn't really about the OS, it's about the software. OS X is fine as a server platform, but it doesn't have the same software and support ecosystem for data center usage. Apple dumped that market with the Xserve because it didn't work for them.
Red Hat/Suse/Oracle etc. all sell tailored solutions for that usage that are Linux specific technology (mostly, some stuff gets ported to other Unix derivatives but most doesn't). Sure Apple could do all that too, but they don't want to. It isn't their market, so why sink money and effort into engineering OS X to do it when they can just buy high quality products ready to go?
What tools is OS X lacking? From my experience most of the development and server tools are available natively on OS X. It lacks support for containers, but that would be a worthy addition, and I would say worth spending money and time on. The rest is already there for the most part. Developing further their server infrastructure would allow Apple to make a play for the corporate market. Any way, it's a silly argument. I thought they ran most of their backend on OS X, it looks like was wrong.
It's not small server stuff like Apache that they are missing. It's stuff like distributed failover, exotic driver support, SAN, management etc. that they are missing. Big data center stuff, the kind of thing companies like Red Hat make.
Those kinds of products are huge investments. Sure Apple might be able to market towards the enterprise, but they simply don't think there is any money to be made. They used to have for instance Xserve that tried to stay afloat in that market, but which made little money. Since they canceled it, Mac OS has only been developed as a small to medium server (which it isn't half bad at). But big time data centers are a different world.
For instance, as a very basic example, does Mac OS support Infiniband or the more exotic high-speed ethernet network interfaces? For Infiniband, the answer is no and in the other case the answer is "kinda, but not really."
My background:7 Xserves still in production here in K-12 education, 1000+ users in OpenDirectory
In the pipeline:Migrating to the new shiny Mac Pros along with OS X Server
Reasons: Thunderbolt 2 connectivity is amazing and works fine to connect FibreChannel RAIDs.
OS X Server: Though it's correct that the GUI got simplified a bit, it's the same server package and complex as it always has been, however easy enough to support. And if configured correctly, a solid workhorse for many scenarios: network accounts for lab use, calendar and contacts server, along with some helper tools it works in heterogene environments fine, supports huge amounts of users in via LDAP..just to name some reasons. for 20 bucks the best server os to support Mac and iOS clients. And because the underlying foundation is UNIX, it's friendly with any networking stuff such as RADIUS for your WP2-Enterprise wi-fi needs..just to name a view.
One thing that is not quite right in the post above: SAN support exists via XSAN.
Why would you assume that? There are a ton of things that linux does better than OS X - and it would be extremely stupid for any company regardless of size to not use the right tool for the job. For example, even IBM uses, sells, and supports Linux instead of AIX or OS/360 on their line of servers and mainframes. I think that your assumption is just really old fashioned.
Internally Apple does use Linux, just as Microsoft uses a blend of OS's - supporting Linux on Azure, for example. I read that they actually use Linux as a host for their Hadoop service on Azure.