I don't think it is entirely fair. While player's interaction with the conversation system hasn't changed much (you pick a line you want to say, our of several available options), complexity of conversation trees, what the game is tracking, various reputation systems tied to conversations has. To say that conversations haven't been experimented with would be as reductive as saying that combat hasn't changed as you still click on enemies and they die.
There's still very little in terms of actual iteration upon the systems. And far too few games tying in both actions AND dialogues into reputation systems (That aren't absolutely barebones where you literally have options that do +/- X reputation - No thought has been put into combinations of things, like extra negative reputation for saying one thing but doing another. Or having other characters react to things you do like companions being concerned when you use too many deception options because they're concerned with how easily and often you lie etc)
What makes them harder to iterate upon is that they are content, rather than systems - in the end it still comes down to a narrative designer writing the content and setting up reactivity and players navigating through this content. Similarly quests structre gets more or less complex depending on a game, but they all work more or less the same way. Because it is the handcrafted content that makes those elements work.
No, they are systems. They're just heavily reliant upon writing. All the options and reactions have to be written and the writing has to enable the multiple options to adhere to the overall narrative. Unlike say, combat where its an entirely closed system where it doesn't matter to anything else other than the combat system what the combat system is doing.
The overall systems for socializing haven't been iterated upon much at all. Besides the curtailing of dialogue lines into the awful radial system that sucks completely (Even when using a controller, it's just worse to use than a simple list...). Things like the emote systems used for multiplayer games could also be used to provide NPC interactions (And there are some easter eggs in a few games where NPC's will react to you using certain emotes). To say nothing of multi-character dialogue systems (Something that would much improve BG3 for instance) where your entire party can be involved in a conversation rather than standing around like an idiot while the MC talks to everyone.
I do think it is a bit unfair - much more work has been put into romances than just sex scenes, but yeah vocal folks who care about this stuff do seem to mostly focus on... hmm... physical aspect of the relationship.
If there has, it hasn't shown.
There's very little in romances in any game. Maybe a few extra lines of dialogue. That's not "Much more work". It's not like characters behave differently when they're involved in a romance, being more protective of their LI or more prone to initiating conversations or doing anything particularly special (Besides maybe some scripted actions)
At best, you get the ability to command your LI to do something like kiss or interact with certain things (I.e. CP2077's update to romances letting you call your LI over to your apartment and then interact with things like a music player, sofa, shower etc. To do some generic animations and make some banal comments)
Where it all comes back to the initial social aspect. The lack of NPC's and especially companions acting like individuals where they go and do their own thing. Instead it's all just cardboard cutouts until you go and talk to them and force them to interact.
