Invisible is not un-targetable. End of story. This is another of those baggage rules people haul with them from previous editions. Even if this is a conscious choice on the devs' part, it just looks bad because this is one of those oft misinterpreted 5e rules. Invisible is unseen not undetected. Now considering the fact that they have made hide a bonus action for everyone the creature turning invisible could also use its bonus action to hide but that isn't happening in the game. They are simply going invisible and often doing other things as well and being rendered un-targetable. Attacks on an invisible target should have disadvantage, per 5e rules, but that only illustrates the fact that they should be targetable. As it is, invisibility is very much overpowered in this game from what it should be based on RAW.
