Why is subshell created by background control operator (&) not displayed under pstree

I understand that when I run exit it terminates my current shell because exit command run in the same shell. I also understand that when I run exit & then original shell will not terminate because & ensures that the command is run in sub-shell resulting that exit will terminate this sub-shell and return back to original shell. But what I do not understand is why commands with and without & looks exactly the same under pstree, in this case sleep 10 and sleep 10 &. 4669 is the PID of bash under which first sleep 10 and then sleep 10 & were issued and following output was obtained from another shell instance during this time: # version without & $ pstree 4669 bash(4669)───sleep(6345) # version with & $ pstree 4669 bash(4669)───sleep(6364) Should't the version with & contain one more spawned sub-shell (e.g. in this case with PID 5555), like this one? bash(4669)───bash(5555)───sleep(6364) PS: Following code was omitted from output of pstree beginning for better readability: systemd(1)───slim(1009)───ck-launch-sessi(1370)───openbox(1551)───/usr/bin/termin(4510)───bash(4518)───screen(4667)───screen(4668)───