I know you are a strong proponent of the idea that players can be trained into certain behaviours by their experiences and I wonder if this is reflected in this - at least in part. In many games, "being good" just gives better rewards - it was a bit of a trope in BG1/2 for example that turning down a monetary quest reward got you both reputation points and magic items worth more than the reward.
Even in BG3 the "good" routes are often better fleshed out and the rewards are pretty similar (getting Minthara as a companion would be counter example, but the general case it a bit uncommon).
I don't disagree with your overall thesis, but there might be a bit of a feedback loop here - "good" routes historically provide better rewards so players choose them more, Devs spend more time on content players are likely to see, players choose the routes with more/better content and so the cycle repeats.
The question is kind of - if the "good" routes required significant content/gameplay sacrifices, would people still choose them to the same extent?
Well, I mean, I cited the example in part because it effectively is a (mild) sacrifice, but it also functionally costs nothing, and yet also adds almost nothing either. Dante (the half-hellhound in question) only shows up very late.
If folks were inclined to experiment just because they felt like it, you'd think the rate would skew at least
somewhat more in the other direction.
It's easier to be nice here, admittedly, but not "only 0.9% of players did the opposite" easy. By comparison, 27.8% had Dante join the crew. That's not even a 1:4 ratio, that's just shy of a 1:
310 ratio.
But, more to your point, yes, I am a strong proponent of that perspective, that players get trained--and in this case, it's
society responsible for most of the training, not the rewards from games, nor (IMO) much from GMs themselves. I don't necessarily buy that "good" endings get more attention though. Star Wars games tend to give plenty to both sides. Even with
Shadowrun: Dragonfall, pretty much every reasonable ending except "f@$# this $#!+, I'm out" is there, including VERY bad endings, and Minthara is a knock against the notion that it's a reliable pattern, indicating at best only a loose pattern. (Consider, for example, Vampire Astarion and God Gale both get lots of rewards and content with no downside...other than making both
much worse people.) Often, I do think being good pays off in the very long-term, but it really is
long term, especially for BG3. (E.g. if you don't kill the tieflings, which IIRC requires saving the grove, you can get some nice magic items like a good robe for Warlocks...but it only comes dozens of hours later unless you're also speedrunning.)
Overall, what data I've seen indicates that it actually does take some pretty serious sacrifices to make good, or at least good-leaning, options be overwhelmed by evil ones. It can't just be mere opportunity cost either, because if it is, most folks will just view that in the "cost for doing business" kind of way--that it's wrong to merely seek the opportunity for benefit. You
will, however, see some movement in the direction of trying to cheat the system--to reap the rewards of "bad" choices while actually still following the "good" path by the game's logic. Minthara's a good example there. It used to be an exploit to save her without siding with the goblins. Larian later patched in an official, if a bit convoluted, way to do it, causing her to join late, at Moonrise Towers.
By and large, that seems to be where most players actually focus their ingenuity: finding ways to, essentially, have their cake and eat it too. To reap as many "important" (not necessarily "powerful") rewards of the bad path, while still being do-gooders.
So, I stand by my original claim. Evidence suggests that, in general, most people choose to be good when given the freedom to do whatever they like with the only consequences being what their GM can inflict upon them. Obviously, "most" isn't "all". There are
absolutely plenty of people who will do whatever evil they can get away with forever, and there are people who will be very strategic about their evil deeds, and there will be people who need a "purge" to get their selfish-jerk@$$ symptoms out of their system before cooling off after.
Exactly why that's the case isn't strictly necessary for my argument. Just that folks tend to choose good over evil in gaming, when the chips are down and they have to make a decision without knowing all the future consequences.