Why are NPC parties so bad?

[Thangil]

I did Grand Cosmos the first time with NPCs and it went well enough. Slow, as expected, but got the job done.

Think the system is perfect where it is? Less efficient than playing with players, which is good because it encourages you to play with others instead.

Then you didn't have the issue I had, where Alphinaud got one-shotted all the time.