Mostly a speculation that the DNA takes over.
I could see this as Kree thing, since they developed it first, but is/was it enhanced or changed by anything that various Inhumans (Universal and earth based) did too?
Yes, the add on was a long time ago too, though, but perhaps more explained over the last few decades.
When you think about the various factors and races, the Alpha Primitives and then Inhumans that singled out into their own breed, some of this was random perhaps (the latter) and some was planned or assisted, the former, to be a slave/work-force.
I just had the most bizarre thought:
They're gone now, I guess, but what if in the past a Bird person mated with an Atlantean. I guess that undersea person would have to be more like Namor or Namora; able to live on land. I'll leave now.