Because we haven't figured out a large body of non-leaky or resilient abstractions yet. If a higher level construct was indeed a true superset of lower level functionality, or was robust enough to be applied to a wide variety of situations, then it would be okay. Right now in web software, because we optimize for development velocity so heavily, the tools a smaller product would reach for are fundamentally different from the tools a very high load service would. Until we can come up with a set of robust higher level abstractions that scale well (or better than they currently do) then we'll be stuck where we're at. Thread pools will saturate, GC will stutter, sockets will timeout, and all sorts of stuff that higher level abstractions do not capture well.

(I kept this post vague but can offer concrete examples of you'd like)