zombie process reap without "wait"

I know if a subprocess does not get reaped properly, it will become a zombie and you can see it by ps command. Also the "wait [pid]" command will wait for subshell running on the background until it finishes and reap it. I have a script like this:

#!/bin/bash
sleep 5 &

tail -f /dev/null

My question is, I don't use wait after sleep 5 & and the parent shell will never terminate because of tail, then why the sleep 5 & will not become a zombie? I see it disappear after finishing in ps, not sure who reaps it?