Nowadays intel is not a reference for C++; the versions found on HPC compilers are not the fastest nor the best following the latest standards.
I don't think commercial/open source is the key here: on the fortran side we have been long suffering from a lot of bugs/regressions with the current versions of intel fortran (with respect to the "latest" features like OOP) -- I would even say that they could be more than we are finding in gfortran.
An explanation could be that they are investing time in supporting their big customers that likely use ancient code bases.
Exactly, that is my point: so we are mainly still in the land of Fortran95, which is not designed for taking advantage of the latest features of the hardware.
Do they have libraries for JSON, network functions, strings, data types (e.g. dict/map, resizeable vector, sorting, linked list, kd-tree...), databases, and all the things that make a programming language useful? Please point me to the library. I'm only aware of numerics.
I guess that's a no then. Well, I better give up on Fortran if I need to load configuration files, sort data, expand an array, take command line parameters, associate some text with the data, or use a hash table, if those aren't reasonable features. Apparently, it's only acceptable to use fortran to load some matrix, process it, then spit something out into a file.
Most of the fortran codes I'm working on would be much simpler with a common libraries to do things like that.
There is a cottage industry that actually do just that: create a wrapper program with nice gui to prepare data file input and read/visualize data file output from an open source fortran model. For example, aermod is an open source air pollutant dispersion model developed by EPA and there are a bunch of commercial GUI wrapper for it because (as you note it) you can't expect scientists without computer science background to compile, prepare data and parse the output of a fortran program. Those thing are hard to do in fortran as a programmer, let alone as a scientist without computer science background.
Regarding C and C++ compilers, for the same reasons most high performance HPC computers use Intel, IBM, PGI compilers instead of plain gcc or clang.