Even the deduction stuff is only because Conan Doyle says so.
Like he could determine cultivars of tobacco from looking at and feeling the ash. And other people never pick up on things that even the reader could. But Holmes does. And then you feel smart, because you even beat Holmes to the punch.
My wife has been watching Bones (again) and I've noticed this a lot there too. It was an episode where the victim was a nine-year-old child. They "digitally reconstructed" the child's appearance and couldn't find a match, but then noticed the hair was dyed, the teeth were veneered, etc. And it's obvious, the child was on the pageant circuit. But they didn't put it together until the answer was given to them. Later on, same thing with a murder weapon. They assumed it was a steel-toe boot, but it's pageant contestants, tap shoes are also available. Etc. etc. And yes, tap shoes were used in the murder.
And there's two reasons for this. This is how smart the writers are. They honestly think this takes significant deductive ability. And making the characters figure it out faster would be implausible. Or. They want the viewers/readers to feel smart. And one way to do that is to make them able to figure out the pieces before the characters. Characters who are the smartest because we're told they're the smartest.