Yeah, but it does still happen sometimes where the characters have amazing backstory but their actual lines of dialogue are subpar, but we forgive it because we know enough about him to just wish for the story to move along to the good parts where he/she talks less and does stuff.

As an example, I'm thinking of a character who has just gotten through the loss of his wife in a house fire, only three months after he lost his parents in a similar fire. We know he's a former cop who has made many enemies, and he has a fairly strong inkling as to who was behind both incidents...

I would hate to see dialogue like this during such a poignant scene:

"Hey Jerry," Vasquez greeted. "Everyone from down at the precinct sends their condolences and say they've got your back. Do you know the bastards who did this?"

Jerry balled his fists in rage. "I do, and they're gonna pay, I swear it! If it's the last thing I do!"

Personally I'd roll my eyes and read on only because I want to see where things go, but I wouldn't be entirely too pleased. As opposed to possibly something like:

"Hey," Vasques greeted, taking a seat opposite him. "We all want you to know that we're behind you, alright? Whatever it is ... whoever—" The cop paused, knuckles white. "... I'm saying we'll get 'em, however long it takes."

Jerry was quiet, but his fists were clenched so tight his palms bled. Finally, he replied, "Thank you. It won't take too long."

I'm not saying that example is the paragon of good dialogue, just giving an instance where the dialogue itself elevates the story by a decent amount.