in my theory, because law and kuzan both are part of revolutionary army. (law being a part of it before the timeskip and kuzan after the timeskip, law joined because of his beef with tenryuubito and doflamingo another theory there and kuzan because he wanted to clean the WG)
unlike ivankov and inazuma who stations themselves in baltigo, law and kuzan works more like kuma. law sticking to being a pirate, in order to achieve his mission of bringing down tenryuubito and doflamingo (and of course pursuing his pirate king dream at the same time) and aokiji working on solo flight.
i would assume law and kuzan are not that of close friends as of the moment, its just that they were given orders by dragon, so they need to cooperate.
it still bothers me that law mentioned that he looking for something "sleeping" in PH (i dont think it is SAD) and aokiji suddenly remembering that he should drop by punk hazard.
i think they just missed each other, with aokiji arriving after law left.
im thinking dragon ordered law to look for aokiji in punk hazard, in order to help him to bring down a yonkou (or whatever, SAD or whatever else he needs to accomplish). its just that, luffy accidentally became an ally.
and yeah, by the end of the saga (or at the end of law's alliance with luffy) aokiji will join law's crew and becomes his vice-captain.