We're here to play, right?

Disclaimer: This is not directed at anyone in particular.  But I recently joined an online PbP game on RPOL, and this situation sorta popped up to remind me how much it annoys me.

What is it with players these days, who get together to play an RPG, but then bog things down by forcing the other players and the GM to have to convince that player's character to go along on the adventure?  It sure happens quite a lot, in my experience.

The players gather, hear a report of some monsters rampaging or of huge piles of gold just lying around in some dank, nasty dungeon.  All the PCs are ready to go.  Except that one.  That one needs to be convinced eight ways from Sunday why it's worth his/her time to bother with fighting the gnolls, or why they should bother travelling to the Lands of Doom to try to find the Emerald Crown of Emperor Colwyn.  And anywhere from 10 minutes to half an hour will be wasted just because that one player's got a bug up his/her ass about needing to be convinced to come.

And the other players never seem to just say, "OK, sit this one out.  We'll manage without a [class X] this time."

Have you seen The Gamers?  Remember that scene where the M-U bites it, the player rolls up a new character, and a couple scenes later in the game you get:
Magic-User: "It looks like your party could use a mage."
Fighter: "You seem trustworthy.  Get into marching order."

I've got no problems with that.

I realize a lot of people have fun getting so deep into the head of their PC that they want to evaluate every decision as the PC would.  But why oh why can't they ever create a PC whose mentality is to just get on with the show and bust some orc heads and loot some tombs?  Why is that so hard?