I see your point, about it being "odd" that a One Piece film hasn't been this big yet due to how massive of a franchise it is, but at the same time, these anime movies based on a TV anime/manga have only recently been putting up numbers like this. Remember that Film Z (the 2012 film) was the second biggest TV anime/manga movie ever at the time. A LOT has changed for this genre since then.
It began with the 2018 Detective Conan film breaking ¥9 billion (seeing a significant jump over its many predecessors), becoming the first TV anime/manga movie to break ¥9 billion, performing much higher than any past movie based on a TV anime/manga. Then, it did ¥9 billion again with the 2019 film.
And as we know, the biggest monster of all, Demon Slayer, went crazy and broke everything in 2020. It was the first movie based on a TV anime/manga to exceed ¥10 billion (not to mention quadrupling that...). It wouldn't have been unusual for it to be the outlier given its record-breaking performance, but just half a year later, Evangelion 3.0+1.0 nearly doubled up on its predecessor and also broke ¥10 billion (with a lot of pushing, but still).
And then we got Jujutsu Kaisen 0 a little over half a year after that going well over ¥10 billion, Detective Conan doing ¥9 billion once again, and now One Piece getting ¥10 billion so fast.
So, this trend of anime movies based on a TV anime/manga doing such high numbers is very recent, barely four years old. If we consider this, when just ¥5 billion was super impressive before 2018 for them, it's understandable, I think, that One Piece never got to ¥10 billion in the past.
¥10 billion requires a lot of casual/general moviegoer appeal, which, until very recently, was something none of them could manage to achieve.
www.fandango.com
