The purpose of level 60+ monsters were to fight for exp, however SE in their infinite wisdom made calculations between your level versus monster level too severe that they can hit you for over your max HP in one hit the higher they go. Since the cap is 50 if they used XI's formula you could exp on level 60-68 monsters no problem, however once you hit 6+ levels apart it starts becoming a massacre.

Much like bosses at 75 were already hitting level 90+ and didn't auto oneshot you (unless it was their gimmick) so in this game the same could have been true, but this is why most bosses in XIV are actually around level 59 since they pose just enough challenge to kill you off easily.