I don't remember where I heard this (meaning it isn't a fact) but I could have sworn it was in one of the games that Argonians need to return to the Black Marsh to have offspring, and that they remain there until a certain age.
That is kind of backed up by the Argonian Lore Page on UESP:
Argonian appearance ranges from reptilian to almost human; this is decided by the amount of Hist sap they ingest as hatchlings, which ceremonially takes place on their Naming Day.
Hist trees are also said to grown only in the innermost parts of the Black Marsh, although the Blackwood Company managed to get one to Leyawiin somehow.