Of course different people have different reasons that may share, and nobody is expected to follow the workflow of someone else (unless they closely collaborate), but I have the impression that the most popular reason is to avoid the mental burden of deciding where to put a note, which in my opinion is a strong reason. As you wrote yourself:
All things fit somewhere else, but are we sure where? So just use the default place and let the decision for another time.
But this should not be the end of it. Unless we are lazy to think for ourselves (and let AI “do” it), later moving mature notes from journals to dedicated pages is an important but separate phase, which I explain here. The thing is that most of the work in dedicated pages can take place without leaving the journal page (and its context), as the right pane, the pop-ups, and the references, are all editable.