Because he's still one? As they said in the chapter, Hazuki is 23, which yeah is legally an adult everywhere you go (I think), but he still has some growing up to do both literally (brains don't finish their last stages of completion till mid 20s for girls and late 20s to early 30s for boys) and figuratively. Naoto isn't that much older, but 1) he is older and therefore had more time to experience things, and 2) he's very much used to dealing with emotions (partially bc of the kids and partially bc of his own experiences).
I didn't get the feeling that Hazuki really had that many experiences with dealing with emotions; heck, we know the guy went and hid his dynamic after that whole thing in elementary. That's not quite a fair example since he was a kid then, but it lasted till he was about to graduate from high school. He's used to hiding (and we see a lot of examples of that), but he's still learning to adequately process emotions lol
He definitely shouldn't have said what he did, and he definitely needs to grow up, but I feel like Hazuki is doing a relatively decent job. Some people that age who are parents have done a lot worse (I've heard stories from someone who comes into frequent contact with young parents lol) He already knows he did something terrible; he just needs to learn from it and...grovel a bit.
Well if you go like that naoto should be a shitty childish parent because he had his daughter while he was still really young, but no, he had to grow up in order to raise his child, and hazuki has to do the same now. He’s a dad, he has responsibilities and he can’t afford to be a child
Hazuki needs to grow up. Bro, ur a dad why u acting like a kid