It's not RARE according to the NFL. The NFL, through Rich McCay, says it happened 230 times last year which is nearly one per regular season game (Technically .84 per regular season game). If it were one every 16 games then there would only have been 1 per week instead of essentially 1 every game.
Which should tell you that the NFL thinks it happens much more often then you do which likely means their definition of a hip drop tackle is much broader than your definition. The NFL does think it's common which means that the way they see the rule, they are banning perfectly reasonable tackles instead of just atual hip drop tackles.