why do we never see anything equivalent to the reconquista, with the andal houses in Dorne (and the other kingdoms of westeros) trying to reconquer Dorne from the rhoynar?

Well, the Andals in Dorne settled in the east, not the west, and thus were right in the middle of the Martells’ orbit. If you mean the First Men kings of the mountains, certainly the Yronwoods tried at various points of time.

As for the other Andal kingdoms – well, the Reach and the Stormlands certainly went to war with the Dornish repeatedly over the years…