Because that's a syscall ;)
https://man7.org/linux/man-pages/man1/flock.1.html is the command line manual.
I would say one good reason is that
waitlock myapp &
JOB_PID=$!
# ... do exclusive work ...
kill $JOB_PID
is a lot easier to use and remember than
(; flock -n 9 || exit 1; # ... commands executed under lock ...; ) 9>/var/lock/mylockfile