Because not everyone wants to try to make their PC come to life or makes decisions for their PC based on what their PC would do. Some people play D&D like it’s a boardgame. For some of them, rules for this kind of thing are necessary.
I understand that you may prefer mechanics that affect a PC's mental space (e.g., thoughts, romance, etc.), this is neither an accurate nor a fair assessment of what people who like social mechanics are looking to get out of them,
@overgeeked. Not at all. It's also worth noting that games where these sort of mechanics are found (e.g., PbtA, Fate, etc.) aren't really all that conducive to "play...like it's a boardgame." So the veiled accusation comes across as ignorant slander rather than any sort of valid observation.
You are playing a fictive role, but since you control (nearly) everything about the role, including the mental space and actions, there can also be a conflict of interest between you as a rational actor controlling the PC (with your own play goals) and the irrational/rational mind of the PC (with their "own" in-game goals, personality, and quirks). This can create a conflict of interest. Obviously some people believe that they don't need or want these sort of mechanics, and that they are above such conflicts of interest: i.e., if you are roleplaying "correctly," then you don't need them.
Additionally some games want roleplay to be about discovering things about your character and their mental space, and these games may use mechanics to cultivate that experience with its various twists and turns. This may be things like having a mental conception of your high school teenager character as straight, but then a heated encounter with somone of their same gender unexpectedly "turns them on." Now you have to roleplay that prompt given to you. What does being turned on by someone of the same gender mean for your character and their sexuality? It's up for you to decide, but it doesn't change that your "straight" character got turned on by someone of the same gender as per the game (i.e., Monsterhearts). But it's a game with clear allusions to teens discovering their sexuality. As Monsterhearts 2 writes:
This move is at the heart of how Monsterhearts understands sexuality, especially teen sexuality. We don’t get to decide what turns us on, or who. Part of your agenda is keeping the story feral, and that means letting your character’s sexuality emerge in all of its confusing and unexpected glory.
These mechanics in Monsterhearts are far less about being "like a boardgame" and far more about simulating the irrationality of the human psyche that may lie outside of our rational agency, self-interest, or self-conception.
I understand and sympathize that such mechanics are not for everyone, but I don't think that belittling them or people who like them as wanting to just play "D&D like it's a boardgame" does these games, your argument, or you any justice.
But then again, I'm of the mind that a return to a two-system setup would be more beneficial than detrimental. Both using the same fundamental chassis, so there's interoperability and relatively easy translation between the two. One specifically geared for low mechanical engagement, extremely straightforward rules, maximal ease of inventing your own rules on the spot, a casual attitude toward any "official" patching of holes in the rules, etc. The other specifically geared for high mechanical engagement, strategic and tactical depth, a strong emphasis on innate balance, built-in support for "simulationism" (which, from multiple fans of the term, I find usually means "system maps pretty close to world" plus "player inputs lead to surprising, yet deterministic, results") and for narrative-driven play. I would not call them "Basic" and "Advanced" because those are super loaded terms, but the fundamental idea is similar. TBH I really don't know what I'd call the "modern day AD&D" because calling it "Modern D&D" is likely to cause confusion due to "Modern" being a term for settings or rules options where stuff like guns and telephones are in the rules, but I'm at a loss for a better alternative word ("Contemporary" would be a candidate...but it starts with the same letter as "Classic," unfortunately).
In general, I tend to agree. A replacement term for "basic" might be something like "Heroic" or "Adventurer," while the more "mechanical engagement, strategic and tactical depth,..." could simply be "Tactical" or "Paragon."