I think the answer is complicated, and I will try and explain. Ultimatly I think it's tied to the whole intimidation factor of trying to get involved in established MMORPGs that plagues a lot of titles as well.
Being a newbie is not a big deal when the game has just come out, and everyone is still learning or has just learned. However when a server has progressed and most of the player base is of very high level, it can become annoying to have people pester you trying to do content in zones that the majority of the server long since abandoned. A lot of players do indeed try and help new players, but when it becomes a constant low-end presence it eventually begins to annoy people.
What's more, when your playing an established game on an established server, the newbies pretty much want to get to the end a LOT quicker than the guys who started out with the game/server, so they can "play for real" where ever the community is currently focused. What this means is that most newbies generally want to be "uplifted" to the endgame where the majority of people are going to be, and that can take a lot of work especially when dealing with someone his is ignorant. What's more doing it leads to people with high level characters who do not know how to actually play at that level due to never having done all the steps in between... of course with few people playing those earlier steps to learn with, it becomes a vicious cycle.
Game designers seem to think that people making new characters constantly helps deal with this problem, and truthfully that's not typically the case. Your typical "lowbie" is generally a character being developed as an alt to help a guild or whatever, is being played by someone who has learned a LOT about the game, and is trying to move through the content quickly, as opposed to slowly experience it and learn something. To a speed leveling "Twink" a genuine newbie is sort of an obstruction.
Not to mention the simple fact that some people (like me) choose to focus on a single character and generally don't focus on alts, and will be found sitting aroung BSing with people if they aren't doing anything to advance their character.
It's honestly a big problem with the genere, and I suspect one of the reasons why Blizzard is launching this whole "Cataclysm" thing is specifically to encourage people to experience the new content with brandy new characters, and create another period where everyone will be learning at the basic level again to draw in new people. Whether this will work or not remains to be seen.
Oh and of course there is another problem as well, MMORPGs are full of jerks. You never know if that "newbie" is a real newbie, or someone who is sitting around asking for help or handouts simply to see if they can get you to do it/twink their character for them. By the numbers that's typically going to be the case oddly enough. You'd be surprised at how many bored players think that kind of thing is amusing. Heck, they might even target someone they know just for lulz. "Oh hey Therumancer, my newbie hunter thanks you for helping him buy his mount... lulz" says the level 80 guildie as he burns 2,000 gold on a new static pet. Most people have a sense of humor about such things, but still nobody wants to get scammed even in jest... and a lot of times it's not something between friends, but involving a stranger who just thinks it's funny.
coz then i can just toy with them
