As we know, the Gulf Stream is one of the most famous and well-studied ocean currents, but it is hardly unique. The Japan Current (Kurushio) is its counterpart in the Pacific, and in the Southern Hemisphere, the Brazil Current, East Australian Current, and Agulhas Current are the mirror-image counterparts. Each of these currents is at the western edge of an ocean gyre. So that’s five gyres.
So, why only one Sargasso Sea? Why are there no similar seas (perhaps with different species) in the other gyres? I would have expected convergent evolution to come into play and create counterparts to the Sargasso Sea, just as geophysics have created counterparts to the Gulf Stream.
I’d guess it has something to do with the contours of the continents. There are patterns across the world, but each system is unique depending on the shape of the land. Like how some places have diurnal tides and others semidiurnal.
It also seems plausible that the Sargasso Sea has nutrients available for sargassum that other currents and gyres don’t. I have read that dust blown of the Sahara has a variety of effects on weather and biology in that general part of the Atlantic. I would guess that this dust contains whatever element restricts growth of sargassum.