it is heavily implied hev suit personnel were a standard in the bm facility. gina and colette's whereabouts (training hologram) support this

the only instance valve used hev personnel is the bodies in xen. so it seems likely the reason there are no traces of these guys outside of xen is merely because of design limitations, oversights or valve just didn't feel like it.

Because they're not sacred gundam armor suits, but mass-produced utility equipment in a hazardous environment where such a thing would be extremely useful and common.