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."