The problem with tabletop roleplaying, as opposed to other forms of tabletop gaming, is the dilemma of what to do when a player is absent. If you're just playing a game about kicking in doors and killing monsters, this isn't much of a problem ... but then, you're really just wargaming, and not roleplaying, right?
If you're roleplaying, you're telling a coherent, long-term story about your characters and their struggles and victories. If Bernadette was here last week, playing her paladin character, and she had just tracked down her arch-enemy the Silent Sorcerer before the group had to quit for the night, and now this week, Bernadette is absent ... well, what do you do? The big showdown between the paladin and the sorcerer is all set up and ready to go, and there's no paladin. Sure, someone else could run Bernadette's character, but it would deprive her of a victory she's been working over the past half-dozen gaming sessions to achieve. Plus, whoever plays Bernadette's character will not play her the same way that Bernadette would. Whether the paladin triumphs or perishes, it won't seem quite as genuine, because Bernadette isn't behind the wheel.
You could just skip the game for a week and play something else, but what happens if Bernadette doesn't show next week, either? Or the week after? Now the gamemaster has to seriously consider writing the paladin out of the story, especially if the rest of the group is about to head off to the Shadow Dimension. You could keep her around as a long-term NPC, but that can get cumbersome. You can leave her behind, but what happens if Bernadette suddenly returns to gaming? How does her paladin catch up with the rest of the team, who are already halfway across the Shadow Dimension?
Whereas if you were all just playing Carcassonne, you could just leave a chair empty at the table.