Because it doesn't. That sort of pleiotropy just is not common, empirically, in humans. Hundreds or thousands of genetic correlations tend to confirm this: there are loads of free lunches. Why is that? A lot of it looks like relaxed selection (eg my other submission shows that our ancestors thousands of years ago appear to have had substantially lower genetic risk of disease because it was decreasing over time, but has reversed), the slowness of selection given the small effect sizes vs the constant incoming stream of new mutations, and the demographic history of humanity of massive expansion from a small starting population based on migration of also tiny groups.
If I understood you correctly, environment pressure relaxed for former traits. But then the new human environment caused the presure for other set of mutations to kick in, even if they're counterproductive for old environment...
There really is no 'hiding' in evolution; if a trait has deleterious effects, even only in specific genetic or environmental contexts, then they are felt. It is this brutally simple force that permits such complex systems to function effectively.
And what if it's true for a short period of time until a new mutation arises that eliminates the double edged sword? Most of our adaptations are really really old. They've had enough time to "perfect" themselves.
What if evolution isn't clean or pretty? What if it's a messy business where a gene arises that gives a small edge, but brings with it a host of problems. It'll still spread because of that edge and it may take millions of years to sort out and eliminate the side-effects.