I think many RPGs focus on combat rules....because noncombat rules are much harder overall.
The reason is that rules can often help combat, it enhances the tactical play that many players enjoy, or it provides a variety of different actions that players can use. Also...the results tend to be more concrete. The enemy is dead and I am not. I hit or I didn't, etc.
Noncombat encounters can be much more subtle in their goals, and one thing I have found is that rules tend to hinder social encounters. Describing a combat action and then rolling some dice magnifies the tension and excitement of combat.
However, saying a few sentences, rolling a die, and then saying a few more lines tends to have the opposite effect. Players will lose the momentum of their conversations and have more trouble acting in character when they break it up with dice rolling.
All so very, very on point. I was trying to put together a system with unified mechanics for all conflict resolution. I was also trying to have relatively robust combat rules. By extension, I needed to have robust non-combat rules.
That opens a MASSIVE can of worms. If everything uses the same mechanics and level of detail, there's a lot to include. Social interaction, of course, but beyond that - investigation, stealth operations/infiltration, puzzles/riddles, trap handling, survival, travel, crafting, etc. Sure, some of those can be rolled up into higher groupings to shorten the list a bit, but then look at the nuances of just one category.
Social interaction - haggling with a merchant, convincing a king to go to war, tricking guards into granting passage, seduction, interrogation, telling a rousing tale to garner favor/respect, bragging contests, insult contests, etc. Let's up the ante and talk about the really complex - person A is trying to convince a king to go to war against kingdom X, person B is worried about convincing the king to hire his smiths to make arms and armor, person C is trying to convince a king to go to war against kingdom X and Y, person D thinks peace is the best option but is in the same adventuring party as A, B, and C so he's trying to subtly sabotage their efforts, person E is the NPC vizier of the king who wants the king to go to war but hates the adventuring party so she wants to finagle the king disliking the party's suggestions but following hers, person F is the queen and she secretly lusts after person C and wants to make sure the outcome sets up a good scenario for her to pursue her desires. And so on.
By no means am I suggesting it's impossible, but it's a heck of a lot of work to bring detail to all aspects of a game. And as Stalker0 said, I worry how detailed social interaction rules would impact, say, my complex example above.