Because that would be closer to what the developers originally planned and similiar to what other developers have done. The game used to have 3 skills for intimidation, charm and reasoning BTW before it was ripped out.
But charisma only does that while strength and intelligence have other purposes. From a maximization standpoint you would never ever get charisma if you could use strength or intelligence to win arguments.
I can plainly see why they condensed it all into one skill. Sometimes you want to use a different line of dialogue. Why force you to
always be a kissass because you picked x instead of z or y?
You are thinking in binaries. A strength and intelligence skill check must not always work. Maybe only a perception skill check would work in certain situations.
They could leave the charisma skill in and use it as a bonus for skill checks (strength, perception, intelligence, whatever). There are many options and all are better than RPS in my opinion.
RPS is indeed a good idea if you play with a friend, but doesn't make much sense if used on NPCs. Just my opinion.
