I always thought that it was because that was bojack’s own spiteful opinion on life, His dad/secretariat was that same voice in his head during “Stupid Piece of Shit” The man Bojack craved to appease while also hated in that oh so special distant-father x son sort of way.
All of the people at the dinner party where bojack’s own mind projected with his guilt, his self-hatred and his own way of thinking about death.
I think Secretariat was a mean spirited nihilist because the man who bojack wanted to be while, feared to become and in some way has become was a mean spirited nihilist.
If it helps, the characters who recognize their flaws and work on improving themselves do get their happy endings not because they are deserving of them, but because they put in the leg work to reach those happy endings.
None of them were gifted with good endings because the author wanted them to have it, same way as the author doesn’t curse anyone with a bad ending they didn’t bring to themselves, and besides, life’s a bitch and then you die, but sometimes life’s a bitch and then you keep living, so there is always the hope of one day getting that happy ending, finding that thing that would help you get your peace. Every day it gets a little easier, but you gotta do it everyday, that’s the hard part.