It is ten times more likely to happen, and when it does happen it annoys ten times as many people. In single-player games you can limit the damage the bug causes by allowing the player to save the game state at key points and resume it later. That is very hard to do in a multi-player game. Say you invite a bunch of friends around to your house, or persuade colleagues to stay late after work to play, and then half-way throughthe game croaks, ruining everyone's enjoyment. You're not going to be very popular, and they're probably not going to agree to play again next time.
Networked games have more ways to go wrong than single-player games.
There are additional failure modes that a single-player games just doesn't have to deal with. The problem of Quadratic Reliability can be somewhat mitigated if you can make your game networking code robust enough that the game can continue even if one or more of the player's machines crash. This is also very hard to do, and most multi-player games do not attempt it.