I can't believe I'm bringing this up again, but...

[Edi]

They are an exceedingly rare poptype, partly because they are rare as such and there are so many other poptypes that can be found on plains (almost everything except the tribal barbarians, lizardmen and sea populations). Most of the other terrain types have a bit more restricted range of poptypes.

The crystal amazon poptype itself exists and can be assignet to a province in a mapfile. I suspect the reason they are so rarely seen is the same reason the trolls/vaettir poptype is rarely seen, they get drowned out by the others. If the poptype determination is anything like magic site determination (i.e. rare ones are rerolled once and only if the reroll is also rare, will there be a rare poptype in a given province), that'd be a very good explanation for why we see so much of the various infantry/archer/militia, infantry/infantry/crossbow etc combinations, they are the common ones.

Edi