That's because in any situation that demands roleplay, in the game with social skills, you can just say "i try to charm/seduce/enchant/intimidate/impress/honor person x" and then roll a die and the die roll can tell you whether you did it or not.
In a game with no social skills, you actually have to roleplay.
The great rebuttal people try to offer to this point is that if you have explicit social-skill rules in a game, that means that it's somehow encouraging people to do more social scenes, whereas D&D with no social skills is just encouraging combat somehow. This is a bit like saying that giving people a ton of tofu will encourage them to eat more meat, while not having tofu on the table means people will forget all about bacon.
In my experience most of the time "the system encourages it" mentality actually causes LESS reliance on roleplaying (and more reliance on 'story points' or having "five dots in diplomacy" or whatever). That's not the sort of encouragement you need or want. Any "encouragement" that makes how well you roleplay the character irrelevant if you roll the wrong die result is not actually encouragement, it's discouragement. It's telling you "don't worry about trying to portray the character, just put your points in the right skills".
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