No offense, but what you are describing isn't a player problem, it's a DM problem.
As the DM it is your responsibility to call for rolls and set the DC. If you aren't asking for social skill rolls, than you can't blame people for dumping Charisma and investing zero skill points in them.
The Barbarian who dumps Charisma and doesn't invest in Bluff/Diplomacy isn't going to beat a DC 20 and very rarely a DC 15. There are a lot of things that are DC 15 or higher.
I disagree. If a player wants to be good at combat and social aspects of the game he needas to build the character properly imo. I run the game I cannot and will not be responsible for players who build characters poorly imo. I can suggest a player not max out con and str. I can`t force them too. So a player to a certain extent is responsible for the way he builds his character.
I can as a dm only do so much. If the party needs to engage in diplomacy I can`t very well lower the DC because the melee types dumped stats. Don`t get me wrong I do ask for rolls and set the DC fairly. I do get the rare player who wants to be both good at combat and social encounters without having attributes igh enough. More often than not they fail at aocial encounters.