My impression is that adult learners tend to struggle with Kanji even though furigana exists because age is a strong identity factor in Japanese culture and furigana is usually used in age-specific contexts.
So a Kanji that might be accompanied by furigana in a shounen manga (geared to kids) typically won't be accompanied by it in a seinen novel (geared towards young adults). Needless to say, adults don't necessarily want to read doraemon (a kids series) as beginner Japanese learners.
Admittedly, once children become teenagers they are expected to be able to read a good amount of everyday kanji words and the use of furigana falls away apart from for unusual readings or less common words, and pretty much entirely for texts aimed at adults. Still, if you're content with quite simple content you can pick up a wonderful amount of language by osmosis with the helping hand of furigana.