I am a mathematician, and some of my generated files take weeks
of computation. Maybe once they are available one no longer
wants to regard them as generated files. Something similar holds
for files generated by software that is not widely available.
Maybe the software is commercial. Maybe it only runs on a
different architecture. Or maybe it was a specially patched version.
In the case of defkeymap.c (where nothing has changed for
over five years), when a key type is added, it is generated
by a private version that is not widely available.
For Linus or whoever makes distributions, who does not possess
the software required to generate defkeymap.c, it is just a
source file.)