. If desire to play a diplomatic your first observation is noticing that there's three skills under that domain: Streetwise, Impersonate, Persuasion. Thus you you raise Int enough to able to tag all three of the skills. From here. You're set to meet the requirement of the skill checks in early game. Continue succeeding at them and you'll reach the thresholds you want.
I cannot speak for anyone else, but for me I note the following:
You correctly state that building and playing a pure diplo character is straight forward. But in a typical RPG you might decide that "hey, my preferred solution to this situation is X" and then you do X. If you suspect that X sometimes requires speechery, you will spec into some speech and then you'll be fine. Maybe sometimes you won't be able to get exactly the perfect solution you wanted, but it's generally not that hard to cover most situations. In CS, you can easily and frequently find yourself in the situation "well, I actually want this character to die, but I also need this speechcraft xp or I will not be able to do speechcraft checks later, therefore I will take the pacifist solution"*.
Basically, in most games your decision making starts from flavor (is my character a character who would resolve this situation in this way?). In CS your decision making will often be mechanical (Am I doing a build that needs the xp from resolving the situation in this way?).
*I don't extort the merchant after recruit evans because I think it's correct, or because I want the 50 credits, I do it because I feel like I need the xp. And considering xp in your characters decision making is very metagamey.