>>39864938First, because they don't seem to comprehend that fiction exists on a broader spectrum than the two extremes of Care Bears and Berserk. Second, they tend to hear "more mature" and immediately conjure a thirteen year old's conception of what mature means, which is just gore, tits, and guns all over the place.
Third and most importantly, they forget or just weren't around for when Pokemon actually was at/near the level of maturity that people tend to want when they make these suggestions. What we have now is very much watered down from the (still child friendly) first few Generations. I mean, think of it this way: A more tonally accurate adaptation of the Gen 1 games would've portrayed Team Rocket as a threatening crime syndicate that did things like run operations out of shady casinos and wasn't afraid to kill a Pokemon. It's not that the games went out of their way to be dark, they just didn't deliberately shy away from the fact that there are wars in the real world, that crime syndicates tend to do nasty stuff, etc.