A large part of it is demographics - "real" games and most anime have an overwhelmingly disproportionate audience of men, generally college-aged or younger. Now, not all men™ fail to understand the issues with consent, but as a competitive gamer and woman who advocates for understanding of the issues with sexual objectification, harassment, and consent, I have been exposed to a lot of people don't understand. Or you could say it's me that doesn't understand; maybe I'm just a preachy SJW and social justice is terrible and men are actually the victims, according to many (hello GamerGate, r/KotakuinAction, 4chan, 2chan). Either way, there's a whole lotta not understanding between me (and most women) and the young men that are over-represented in gaming/anime communities. It has nothing to do with assumptions and everything to do with an overwhelming number of personal experiences from my primary hobbies in life. To say that I feel marginalized and unwelcome in the gaming community at large, both for being a woman and for trying to combat what I perceive to be issues, is an understatement.
After some of the arguments I've seen in this thread, I'm not entirely certain that everyone does understand the issue with slavery in reality. There are a lot of people who seem to think that if a child consents to slavery, it's OK to enslave them. And that ties back into issues of people not appreciating or valuing what consent really means; we are still fighting as a society about that. To me, suggesting that a 15-year-old girl can genuinely consent to slavery is downright disturbing. One can't genuinely consent to sex, after all, in most jurisdictions. The recent Hollywood scandal offers a fantastic example of how murky coerced consent is, especially - Louis CK asked adult women if he could masturbate in front of them, and they said yes, which is technically consent. But as a leader of industry, so to speak, he held power over them and they did it for the sake of their careers, not because they had any desire to help him get off. He himself understood that was wrong and apologized for it. That is value for consent. Saying that a 15-year-old girl isn't a baby and knows enough about the world to consent to slavery, that because she wants to belong to Elias it's OK, is not valuing consent.
Am I supposed to trust that people who advocate that a 15-year-old girl can provide consent, whether to sex or slavery, in fiction, know that actually that's wrong and they can't consent in reality? That they can see it as romantic in fiction, but won't see it as romantic in reality? My experiences tell me that distinction between reality and fiction isn't actually there, that the people who support this relationship don't understand why it would be wrong in reality either.