Hold on now, your argument is getting a bit off-base.
First of all, in terms of itemization there is no reason why Karma can't build
(in fact, it has great synergy with her R+E). Since we're talking about Karma in the support role specifically, this is actually a strong choice. In addition, Karma has item options that just don't viably exist for Nautilus as a support (like
)
As far as being useful across the entire game, you can't downplay the strengths Nautilus has, for sure. However, Karma is also useful across the entire game, it's just that her playstyle--and even role in teamfights--shifts in the mid-to-late game.
Early to mid game, Karma is a strong poke support. However, when the teamfighting phase starts, Karma transitions into pokes and strong group engaging. Karma excels at getting her whole team into a fight at once. She also helps her carries kite enemies with her slows, roots, shields, etc. It's different than the standard "disrupt the enemy lines" engagement, but it is useful and effective in its own right.