That's untrue.

You only have to consider single-player RPGs to see why.  How much grinding did you do in the best single-player RPGs you've played?  If you've ever played pen-and-paper RPGs you won't have done any grinding either.

Grinding exists because players can consume original content faster than the developers can produce it - not because it HAS to exist.  It's a replacement for meaningful content - a cheap way to keep players feeling like they're doing something whilst they wait for the next update.  It's also widely used as a means to inflate play-through time - so 2 hours of interesting gameplay becomes 102 hours of play time (100 of which is mindless grinding).  It is NOT something that MUST exist just because something's tagged as an RPG.