Well, in the case of Eigen it's an entirely header based library, so the compiler can optimize some things. Not to mention the template based meta-programming strategies [1] Eigen itself uses to simplify and optimize during compilation (including vectorization and loop unrolling). The fact that this kind of infrastructure can be built up in a library in C++, i.e. it doesn't have to be baked into the compiler, is a huge plus I think relative to Fortran.

[1] https://eigen.tuxfamily.org/dox/TopicInsideEigenExample.html