Why can't interviews be better?

So, I recently gave interview for a trading firm. Got rejected after the 1st round(live coding round) itself. All this time (~3yrs of my career, I have only been rejected when my resume doesn't get shortlisted). Once it gets shortlisted, I have always cleared all rounds. I guess there's 1st time for everything.

I just got feedback that my answers didn't have much technical depth (when the person asked casually about things on my resume). I mean, what? I gave overview on the point asked, if you wanted depth, you can ask for it, I am not gonna deny! What's this feeling of entitlement that interview answers should be the way they expect, the code should be the way they would've solved. Why? We are 2 people who could potentially work together, why can't interviewers come to an interview with the perspective of working together? For all the interviews I have cleared till date, everyone of them came into the interview with the focus that, hey, I want this guy. It's gonna be a conversation and I wanna see if it's fun working with him

While I am interviewing, I am also evaluating the right fit. Something did irk me when he interviewed. I mean, this guy is head of the department, and he's coming to take the 1st round? Really?

I guess, all that happens that is for good. I know I have lot to improve upon, I am gonna work on it. This rejection just didn't seem fair as I personally would love to know more and more based on the explanation given