I think i built something similar after herdr was released but i can say the reason is at least

tmux is definitely usable but moving between workspaces/repos can be annoying if youre managing mahy many things at once. and difficult to organize. more over having agents manage other agents via tmux can lead to garbled input/output

i havent used codex. claude subagents i think arent the right comparison - the closer comparison would be claude managed agents. but i imagine one reason herdr might be better would be that you’re not stuck in the anthropic ecosystem