Is it just me or does writing advice dumbs down books?

Maybe it's just me but when I read writing advice like "cut it short so that the reader's attention doesn't drift" and "make it simple" and "show don't tell" it all sounds to me like "readers are so stupid - you have to spell it out for them or they won't understand". It's like we have to dumb down our books for readers to get them. But isn't it a bit insulting? Don't we underestimate the readers when we do it? And what's wrong with adverbs, anyway? Can't people paint their own picture in their mind of what the character is doing if we use them instead of spelling out everything? They're not stupid. If anything, I think we should make it more challenging. I, for one, love a book that challenges me. I like adverbs. I like passive voice. I like long, complicated sentences. My favourite (world-famous, best-selling, loved, admired, worshipped) authors love them too and their readers and legions of fans don't have problems with any of it, because, in the end, it's The. Story. That. Counts.