BECAUSE ONE PIECE ISNT GOING ANYWHERE IF THEY DONT FIND ONE PIECE, HURRR, WOULDNT THE LORD OF THE RINGS WOULD BE AWESOME IF IT WAS JUST ONE MOVIE WITH FRODO DROPPING THE RING AND NO WARS IN THE MIDDLE? HURRR.
each arc is pretty much a story on its own, Arlong arc is about 10 episodes and was a quite well told story that ended well, the story of arlong is over, etc, its just like jojo, the main plot is the jojo lineage defeating dio and its manifestations, but the whole point is just showing awesome battles and brick shitting moments.
for example, jojo isnt over yet (LOL PLOT DOESNT GO ANYWHERE) but many internal plots are already over, Part 5 with Giorno could have well been skipped and the plot wouldnt have been affected, (except for polnareffs fate) but it was there and it was awesome.
its the same with one piece, Vivi and arabasta was a plot by tself, and its its already over, the Kalgara&Norland plot was made of manly tears, its already over, etc.