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A lot of people don't know that the original japanese pokemon red and green not only had different pokemon sprites, but also different overworld graphics(pic related, looks much better imo, lots more contrast). Its easier to tell in this version that they are supposed to be bollards.
With that said you should accept for the most part they are meant to be seen more so as a symbolic delineation of the bounds of playable space rather than actual objects that truly exist in the world. You will find that they are depicted even in the ocean where they would make absolutely no logical sense to be, they are merely there so you don't have to deal with invisible walls.