Mass fabs are more efficient; however:
There is not always space to make them
Planning where to build them requires apm and time investment, whereas for RAS SACUs you can just hit the repeat button and forget about it.
If mass fabs didn't require space and I could build an infinite number by pushing a single button once, then I wouldn't care that Seraphim have no RAS SACUs. And yes, I know about templates, but sometimes it is even difficult to find space for templates. And again, that takes time—not a single mouse click like the repeat build button for SACUs.
On space-limited maps, Sera is at an incontrovertible and distinct disadvantage when it comes to very late game eco.
Keep in mind RAS SACUs can also protect themselves from t3 artillery by building and/or assisting shields and/or moving underwater. They can also run from incoming mobile threats. Mass fabs can do none of those things.
