Because there are various versions of the program.
There's no "standard" that describes which options
netcat has to support, unlike many other standard utilities that are specified in POSIX. You have
netcat versions from OpenBSD, FreeBSD, the GNU netcat, et cetera.
For the OpenBSD variant the manpage notes that:
There is no -c or -e option in this netcat, but you still can execute a
command after connection being established by redirecting file descriptors.
Be cautious here because opening a port and let anyone connected execute
arbitrary command on your site is DANGEROUS. If you really need to do this,
here is an example:
On ‘server’ side:
$ rm -f /tmp/f; mkfifo /tmp/f
$ cat /tmp/f | /bin/sh -i 2>&1 | nc -l 127.0.0.1 1234 > /tmp/f
On ‘client’ side:
$ nc host.example.com 1234
$ (shell prompt from host.example.com)
By doing this, you create a fifo at /tmp/f and make nc listen at port 1234
of address 127.0.0.1 on ‘server’ side, when a ‘client’ establishes a
connection successfully to that port, /bin/sh gets executed on ‘server’
side and the shell prompt is given to ‘client’ side.
When connection is terminated, nc quits as well. Use -k if you want it keep
listening, but if the command quits this option won't restart it or keep nc
running. Also don't forget to remove the file descriptor once you don't
need it anymore:
$ rm -f /tmp/f