You answer as if OP didn't understand the default behavior of OneDrive. They understand it perfectly. They are asking why it sucks?
The current functionality works entirely against the best interests of the user and that it is not at all intuitive.
On my PC's system, Windows saves files in a hierarchy that includes OneDrive by default, even though I never installed the OneDrive app. Even newly created files somebody shared to my pc via bluetooth or some other means automatically save to the cloud.
The UI is a mess. I have a Terabyte of paid storage, yet if I want to share anything with a non-technical person I have to upload those files to a more straightforward venue, unless I want to create a tutorial just for them on how to use it.
I am currently need to share photos people have sent me of a close relative who just died. It would be nice to save them all to a dedicated OneDrive folder and just send everybody the link where they can see my images and share their own, but if I grant them that access, they are also able to subtract files from the self-same folder, potentially deleting them not only fromsthe cloud but from my local devices when syncing occurs.
The photo management is abysmal. The captioning tool is buried in a menu and the captions it creates are only visible in certain views, if the user even notices them, since they appear written in small default font in a tiny field.
This doesn't even begin to address the issues with creating albums, which can be shared, but not downloaded. The recipient has to individually select each photo they want to save locally. MS didn't even include a "select all" option.
You say you are an advisor. It is perfectly fine to add a detailed explanation advising lurkers as to how OneDrive works (or rather, doesn't work) after answering OP's question, "Why is OneDrive so terrible?" but you ignored the question so as to be the first person who answered, stealing focus from the original problem for clicks.