Because even if the player is unskilled in speech, it can be that his character is a master of diplomacy, because he isn't good in talking he will have a harder time than simply just roll the dice, but still he will have achance to succeed.
I want to encourage roleplay and dialogues, not to banish people who simply can't do it better. Anyone has a chance to accomplish anything.
This way its easier for roleplayers, taugher for those people who are not so good in it/or are beginners, but definetly it isn't impossible.
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As long as the players who are not so good in person are able to succeed with characters that are excellent diplomats (for example).