Because the purpose of Hinata's confession was for her benefit. I firmly do not believe Hinata expects Naruto to feel the same way about her, she told him because she thought she was never going to get the chance to again and that she would die without relieving that burden off herself, that is what she meant by "I'm just being selfish". I think it's unlikely it will ever be addressed again.
I agree. As I've said several times before, I can't imagine what Hinata could have to say that hasn't already been said and Naruto has chosen not to answer. And while I believe she does love Naruto, I think it's more in the sense of what he's done for her -- how he has changed her life. She has romantic feelings for him too, but as you said, I don't believe she expects Naruto to return them.
And I think people are highly misguided on what the term 'asexual' actually means, uninterested and asexual are two completely different things. As for Sasuke's thank you to Sakura, I don't see it as "thank you for loving me", so much as "thank you, but I don't feel the same way".
Although we cannot know for certain how Sasuke feels about romantic relationships and sex, his characterization has consistently lacked any normal display of sexual attraction to another person and zero interest in sex. That's the definition of asexual, and why people often use it to describe him.
Unlike Naruto, we've never seen Sasuke trying to peek at a naked girl at the hot springs. We've never seen him notice a girl's behind while she's bent over (as Naruto did when he saw Sakura bending over to look at Sai's drawings) and express jealousy. We've never seen Sasuke ask a girl for a date. Or express the slightest interest in romantic relationships. Even Sai, a guy who had his feelings systematically stripped from his consciousness, has done that much, and played an integral part in Naruto and Sakura's relationship of his own volition.
The other thing that's funny is Road to Ninja. In that movie -- where people were the opposite of themselves -- Sasuke was a womanizer who spent his time schmoozing all the girls. The antithesis of that would be asexuality.
It could be that Sasuke simply has not yet met anyone who matters to him more than his agenda, and at some point later he will display normal sexuality. But at this point, he does seem to fit the asexual model.