Sorry, and how is that different for any other maker? Nvidia Pascal series are now over two years old for example, so all these new laptops using Nvidia graphics are essentially using 2 year old tech as well. CPU situation is similar, since Intel has basically kept the same architecture (Skylake) with minor improvements for over two years as they are seeing considerable troubles with newer chips production.
In addition, Apple's problem is that they traditionally rely on higher-tier Intel CPUs (with higher-end GPU package), but Intel has been slow in delivering those. The Kaby Lake R (which is nothing more but a 45W CPU reconfigured for less energy consumption) comes with the Iris 620 GPU thats up to 50% slower than the Iris Plus 650 Apple uses for their dual core CPUs. Similar story was with their 2016 model — Skylake with Iris Pro never had high enough availability so they had to wait until better GPU options were available. All in all, its difficult to introduce new hardware with regularity if there is nothing on the market to replace it with. Don't forget that Apple needs higher volumes of the components as well — since they sell more of a certain laptop model than most other laptop makers, while using premium components. Other makers, say, Dell, might sell more computers overall, but most of those will be budget versions that can use cheaper and more readily available hardware (like i3-class CPUs). Its easier for Dell to source high-end CPUs if they only sell a handful of workstations using those CPUs. Apple in contrast needs a steady supply of very high volumes of those chips.
And finally, if you are referring to Coffee Lake, well, those CPUs only really became available around two weeks ago, despite launch in March. My hope is that Apple updates their line coming Monday, although industry sources say this is unlikely.
