They are compatible but that's not the point.
If it were included it would instantly become a LGPL hard-fork because of any subsequently added line of code, if not by "virality" of the glibc license, at least because any glibc author code addition would be LGPL, per GNU project policy/ideology.
Also also this would he a hard bar to pass: https://sourceware.org/glibc/wiki/CopyrightFSForDisclaim
As I recall this is what prevented Apple from contributing C blocks† back to upstream GCC.
† https://github.com/lloeki/cblocks-clobj