Because there is never any focus on her. It's always her and someone else.
That could be said about Yukinoshita too. That doesn't make her only reason to exist to make the others seem more interesting. She does so much that influence Hachiman and Yukinoshita, the actions of others etc. that I don't see how that makes her that. There isn't any direct focus on her, but that's reasonable. She is the kind of person that has to be with others, that wants to help others etc.. It makes sense to put her into that position, especially with Hachiman's point of view.
Hayato is a walking trope.
How so? He appears to be perfect on the surface because that's who he wants to become. There were 2 scenes in particular that sad much about him as a person with only very few time. One at the end of the last arc where he almost beat Hachiman up and the other one being at the end of the camp arc.
Sadly I haven't seen Hyouka yet so I can't really comment on that. But keep in mind that it's the point that Hachiman doesn't have a tragic past like Yukinoshita. Like I mentioned before, some people just work this way. Little things can already turn them away from people. Giving the contrast with Yukinoshita's past really showed that. Some human's are more likely to become like that, others less. The whole point of the show is to show how ridiculous, but also how human Hachiman's attitude really is, not what he makes it out to be. That's why I think the show does a brilliant job at executing it. It uses the romcom setting to create a human story of the characters that it wouldn't be able to do without. The romcom is what Hachiman would want to have, only that he can't. That's why the romcom setting is such an important part of the narrative.
in a nights sleep every time
Keep in mind that there were several timeskips after arcs. That said, Hachiman is thek ind of person who tries to hide his inner feelings, at least the ones he cares about, as much as possible. That's why when we see him acting different than usual outside it's such a big deal. Like I said, the last arc's climax scene showed just how much he was affected by what he did. His actions after being affected like that, or how, what and to who he talks also show much he is affected, but on a more subtle level.
Indeed, he doesn't have to become completely social. What I ment is being able to put down his delusion and facade and be able to interact on an honest level with people. It would be fine, probably the only possible thing, if he were only able to completely do that around Yukinoshita and Yuigahama.
The shifting focus into comedy also says something. Hachiman narrates the show. Therefore if there's comedy, approaches a usual romcom, what Hachiman, to an extent, wishes for. Also showing the small crack of change in him.
Lack of character building? What? Either you missing both the subtle things and the obvious ones or I just can't understand what you mean.
And pretty much every scene in the show served a purpose, be it for further characerisation, building of relations, thematic purpose or whatever.
And again, it's not a show (yet) about the developement of the characters. It's about understanding who they are, as who they see themselves, as who others see them, what they want, etc.. The developement is yet to come. Showing developement of the character before fully understanding them serves little purpose in general and absoluetely none in the context of this show. It would loose so much affect on the viewer if that were the case.
Something you might want to keep in mind is that the main audience of both the source material (light novel) and the anime are teenagers. Irresponsible, insecure teenagers struggling in the world. The show/light novel will ultimately (most likely) try to show just how wrong and stupid Hachiman and Yukinoshita's attitudes are. Many teenagers go through the same stages as both of them go. Just look at how many people on /r/anime used to say how they related to Hachiman. Many people even said they were literally them. But that understanding, rellatedness to the characters only work because of all building of the world, the characters and their relations. That's why when Hachiman will eventually fully develope it will have so, so much more impact on them, and help them come to terms with it.
You seem to think of the first season as one whole story, but it's only a part of one. A part of one which I already find absoluetely stunning on it's own, even without a second season. But a second season would definitely also clear up your complaint for a lack of character developement (which again I don't think is true).