Designers and implementors are traditionally under a very different set of constraints than "average users" (who in this case are programmers), though their goals do align. But this leads to very different set of assumptions, optimizations, and tools being available. Things that would generally be overkill are suddenly very viable and worthwhile goals to achieve (e.g. micro-optimizations add up repeatedly so they're very worth it, vendoring is much more useful to control dependencies/scope of bugs), and vice versa.
So I wouldn't be extremely miffed if you look at libstdc++ or libcxx or <whatever> and find it somewhat unkempt or gross compared to average code. It's often very much not-average. In fact it's likely given careful understanding of the constraints that code exists under -- you'd come up with similar results.
It can be misleading to think of standard header code (especially template code) as "idiomatic C++". It'd be better described as "idiomatic standard-library-implementor C++", since the folks implementing stuff in std are operating under a whole set of constraints that don't affect ordinary C++ programmers. Arbitrary user-defined symbols that could be in scope, user-provided types that could have overloaded operator-comma, etc.