There was no agonizing at all. It's a long form (and video) needing all founders discussion/input and the questions are tailored and formatted for YCs requirements.
Sending the application to other people or posting publicly isn't going to help understand why YC specifically rejected it either.
It's reasonable to want more feedback, but given YC's model and applicant volume, it's also reasonable for them to give most applicants a generic decline, as well.
I'd say your reward for the effort was the chance at acceptance, plus the benefits of the exercise – having necessary pitching discussions with your cofounders.
Why focus specifically on why the YC process passed on you? Lots of other people can give feedback that will reasonably approximate YC's – and perhaps more importantly, will let you know what other funders/mentors/customers would think.
The keen desire for a YC-specific-explanation is itself a potential reason they might avoid giving detailed feedback... even a few words might encourage some teams to obsess over YC's estimation (and perhaps keep trying to 'clarify'/'explain' their application) rather than moving forward on other opportunities. Sometimes, a simple 'no' is a favor, too.
Sending the application to other people or posting publicly isn't going to help understand why YC specifically rejected it either.
Do you think feedback is an unreasonable request?