I learned probably 90% of my Spanish through Duolingo. My reading is good enough that I can usually follow along with Spanish news articles and Spanish spoken at a moderate pace. (So almost none of it, haha) I have hearing comprehension problems with English as well though, so that’s not Duolingo’s fault.
I’m definitely not fluent, but it’s not like I wouldn’t know what to do if someone handed me a form in Spanish, either.
Overall it’s just the repetition that matters. I don’t think I would know any less Spanish if I’d spent 20-30 minutes every day for the past 2+ years using a different app to learn.
By the time you’re old enough to ask “why,” they’ve subjected you to enough propaganda to ensure it doesn’t even occur to you to do so.