Kubo has stated multiple times that he makes his assistant do the background. Meaning he voluntarily chooses to not have big backgrounds. He has always said his focus is on his characters art and that big backgrounds are distracting.
Case in point, S8's early chapters where the background overpowered the characters was quite literally the biggest complaint here on this forum.
A lot of people complain that One Piece's art is often too crowded and hard to follow.
If you want characters as detailed. Then here. Kubo is right up there with the best of the best when it comes to character details and expressions;
You do know that there is not "right" or "wrong" with artistic choices, I assume? His lack of background may not suit your style or taste but its not wrong in any form or manner. His focus has always been focused on his character art.
Even when he did do backgrounds, he'd often whiten out the backgrounds around the characters much more than what other mangaka usually do.