If Kobo did that, then you'd have the opposite problem: people need to be able to change the font for accessibility, and if the font menu doesn't override all the fonts, then you may be stuck with some unreadable text if the publisher selected a bad font for certain paragraph styles. There's no universal solution except to edit the book beforehand with the fonts you want. If you like randomly changing the font for fun, then you have a lot of extra work ahead of you.
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