Why can't this program be installed manually like every other program ever?
Because it's not been maintained for about three years. Gitlab has moved on and is no longer compatible with the latest binary version.
I'm also a bit confused, why there's no standalone target. Maybe leanlabs does just dump the docker container for standalone targets.
It's your choice as the dev.
I'm flattered, but as stated before, i'm not the developer of kanban. Kanban is dead for quite a while now, i just did some adjustments to make it work for myself and shared the code, so others can profit from it. And i'm up for helping people with their installation. Yet i'm not an official developer of kanban.