[vishnu]

From the startx manpage:

Arguments immediately following the startx command are used to start a client in the same manner as xinit(1).  The special  argument '--' marks the end of client arguments and the beginning of server options.

You need some way to differentiate client options from server options, so why not use the two dashes?  It's not very different from the way things like 'rm' and 'grep' use two dashes to mark the "end of options" when necessary (e.g., while filenames for 'rm' or patterns for 'grep' start with a dash).

Well I did not KNOW dat! 🤪 😎 😁 🤣