That's because an "IT engineer" is like an "engineer in chemistry". You have so many sub-disciplines that a given engineer can't be expected to know everything. Nobody is surprised if an organic chemist doesn't know much about heat exchanger optimisation, or a chemical process engineer is confused by organic synthesis.

In the same way, I've met devops guys that know surprisingly (to me) little about the intricacies of hand writing multi-threaded high performance calculation code. In the other hand, they're wizards at stuff like load balancing and optimising how we use sub-processes for I/O, CPU-bound or memory bound tasks. You would think the two are very similar, but even then it quickly becomes clear that people really become specialists in their field.