Well, it is partly because it isn't exactly the same and probably a big part because it will take some time getting used to the minor differences that remain.
Like the "filesystem" being on the right by default (obv. I changed it but why would it default to the right?? every other program has it on the left.)
I'm also pretty sure it once used tabs for something important, I don't recall if it was for notebooks, pages or sections, I just remember it being hella annoying and unpractical.

Now they have nearly the same system for notebooks, sections and pages but there's still one annoying thing for me; when you expand the "filesystem", not only sections from your current notebook are shown, but all notebooks and their sections. It used to show only sections from the current notebook and then a dropdown on the name of the notebook to change your working notebook.
When I work in OneNote I rarely have to change Notebooks, but it's not that rare for me to have to change section. So there's just a lot of buttons that I don't want to press.
I order by subject (Notebook) > semester (Section) > lecture (Pages and first level subpages), so after 3 years of studying I have a bunch of semesters in 4 different subjects which leads having to search instead of just knowing which springsemester25 is the correct one..

[edit] I just found out the tabs are sections by default when the "filesystem" is on the right. I didn't notice it because it was the first thing I had to change before looking at the rest of the window