I agree that with more people the difficulty increases with the number of people and the quality of the rewards should reflect that. But in all MMOs I have played so far that wasn't the only factor that made raids more difficult. The complexity and number of mechanics, the requirements in terms of dps, hps, cooldown usage etc. were always much higher in raids then in dungeons or single player content. I would even argue that they could make the individual requirements a little bit higher in smaller group or single player content to offset the difficulty that comes with the coordination and logistics requirements of raids.
As of now when it comes to Wildstar so far for non-raiders in terms of PVE I have seen dailies, 5 men dungeons, shiphands and adventures, which all seems to be just a stepping stone to eventually get to raids and not an alternative long term progression path for non-raiders. I hope I'm wrong and they offer some kind of meaningful progression for someone like myself who isn't interested in raiding atm, but until I see it Carbine won't get my money. That's unfortunate because there are many things I like about Wildstar, but everything I hear and see tells me it is a game that doesn't cater to me at all.Second question: Why shouldn't they? Has someone claimed that there shouldn't be difficult single player content?