It mostly comes down to the supply chain, from what I can see. Apple is already pushing their luck with the 5nm node, their current strategy is pretty much just keeping everyone else off the cutting edge technology so they can monopolize TSMC's manufacturing chops. They could pivot to servers, but that would cannibalize their plans for the Mac and probably force them back onto the 7nm node, which would probably throw away the majority of their bragging points right now.
Plus, if history has shown us anything, people simply don't want Macs in the datacenter. xServe gave people the option, but that quickly went the way of Itanium after a year or two. Apple's history of "it's not a bug, it's a feature" and "you're holding it wrong" doesn't exactly appeal to corporate buyers who want to set-and-forget a system and have a number to call when it breaks.
