The problems they solve might go beyond "static linking", but are accidental complexity problems that doesn't go beyond namespacing.

With proper namespacing support, one could trivially build "with GCC 11 and its associated standard libraries and toolchain" on a CI service that ships with "an older version of GCC".

Ideally, one would just need a local folder with the GCC11 and its dependencies, and at most an ENV entry for where to pick up deps (optimally not even that, the GCC11 binary should give precedence to the local versions within the same folder by default).

But, no, instead we need to juggle 20 folder locations, PATH and EVN variables, burn-in build paths into libraries and executables, and so on...

This.

I would also point out that containers handle also other critical features like it's virtual network, which obviously goes way beyond what may be simplistically described as "static linking".