Some don't read it because of having disabilities or difficulties in reading and others for being lazy, mostly laziness...simply not into reading...even if is small texts.
When a mangá is adapted into anime, usually there is many problems, such being faithful to the manga, having only what's important, it's a rare jewel when it is faithful, it's even more rare when in terms of storyline and character is better than the mangá (but in this case, it's more of matter of opinion), usually I prefer the manga thousand times more than it's animated adaptation, the only exception to me when I prefer the adapted anime than the mangá was Angelic Layer.
One of my favourite animes of all time, I re-watched so many times, 4 times, I believe.
Also there is animes are adapted into mangás, in these cases, I just watch the anime, since the original story comes from the anime anyway and sometimes the adapted mangá changes too much and back away too much from the original story.
I give you few mangás adapted into anime that were horrible animated adaptations: Fruits Basket (way too many plot points left out even from where it ended and changed some plot points, etc, it didn't go to the end, many of us wish a remake and this time faithful to the mangá), Black Cat (they changed the sequences and good amount of the plot, it could be such a good anime), etc.
I'm fine with few changes, as long as is faithful to the plot such as Fullmetal Alchemist: Brotherhood (you don't even have to bother in reading mangá, unless you want more few details), Death Note and some others with no fillers or fillers arcs that many times changes or mixes with the plot to the point of making very confusing instead unlike some animes I could mention *cough*Naruto*cough*Bleach*cough*One Piece*cough*Fairy Tail*cough*Yu-Gi-Oh! Duel Monsters*cough*etc*cough*
that's it for today :p