Why are some packages from yocto not built

[Alexander Larsson <alexl redhat com>]

Looking over the manifest, are there any particular reason that these
modules could not be used as-is from yocto:

gperf
libpng
libmxl2
libxslt
iso-codes
libarchive

These are not really part of the gnome release cycle as such and do not
really need to be continuously rebuilt. They also seem to have more or
less the same version in yocto 1.7 as in the manifest.