shows the jobs managed by the current shell. Your script runs inside its own shell, so
jobs there will only show jobs managed by the script's shell...
To see this in action, run
sleep 60 &
To see information about "jobs" started before a shell script, inside the script, you need to treat them like regular processes, and use
ps etc. You can limit yourself to processes started by the parent shell (the shell from which the script was started, if any) by using the
$PPID variable inside the script, and looking for processes sharing the same parent PID.