To give this another go...
The sense of wonder. That's it. Nautical stories are awesome. But they don't generally have the same sense of wonder you find in sci-fi stories. Spelljammer takes the nautical and adds that sense of wonder...by putting it
out there. Both literally and figuratively. Anyone read Clarke's Rendezvous with Rama? Niven's Ringworld? Pohl's Gateway? Banks' Culture series? Perry Rhodan? You can't do any of that with a planet-bound pirate game.
Why not just run a planet-bound pirate game? Because doing it in space and/or the Astral Sea opens up all the myriad possibilities you can't do with a planet-bound pirate game. Sure, you could mangle and kludge some of those elements into a pirate game, reduce them, make them smaller to fit the smaller sandbox, but what you lose in that process is exactly why you'd not do so. What you lose is the point. What you gain by going
out there is the point.
At a certain point this feels like explaining why you love chocolate to someone who doesn't like chocolate.
For those who might not know the term:
en.wikipedia.org