I play both Necro and Sorc a lot and have to say that on an early-to-mid tier level of gameplay, sorc is significantly easier to play and that's why you might see higher DPS output from it. Sorc also shines a lot in pure single target DPS.
However, when it comes to very end-game content where players start reaching a high skill ceiling, I actually found Necro to be not so difficult compared to sorc. One of the main differences in the playstyle is that Sorc has a very strong spammable, so overcasting dots on a sorc is a DPS loss. While it's vice versa for the Necro. Necro has a weak spammable but strong dots, so overcasting dots on a necro is okay and actually preferable. I found on sorc that I need to pay high attention to not refresh dots too early or dps suffers, while on a necro you can refresh early without really any penalty and trying to use the sorc playstyle of refreshing at the last moment actually hurt my necro DPS a lot.
Either way, I would say that Necro is definitely a bit more brainmelting to begin with, especially if going for a dynamic rotation.
So Necro shines wherever a lot of AoE damage is required. Necro should perform incredibly well in trash fights and some trials (such as vCR where I keep seeing Necros often at the top of the list, also boss fights like vHoF Triplets, when they're stacked, I've seen a 198k DPS from a Necro while highest from a sorc I saw was around 150k. This was in my scorepush group but we were nowhere near the top players. You would probably see a lot higher numbers from the best of the best)
There was also mention of vAS and yes, vAS is one of the weakest trials for Necros as the vulnerable phases of the boss are in 15 (ish?) seconds and dots are not really worth refreshing. Classes like sorcs (and nightblades especially) always outperformed in vAS by a significant amount. Top logs will likely have a lot of nightblades on the top, but sorcs are very slightly behind with their strong single target. However you will most often see only one sorc in optimized group, which will also likely be the interrupter at the medium level scorepushing, so will lose some DPS. While at the top level scores you would likely play without an interrupter.
Lastly Necros will also lose a little bit of DPS by having to use Collosus instead of Destro Ult (It makes them lose a little bit of DPS but buffs the whole group's DPS significantly, so it's very worth). And also you would often have a necro in a support set, such as EC as they're very good at keeping it up.
I recall someone saying that there are other sources of Major Vulnerability which isn't true. Major Vuln. only comes from Necros and I suppose Kynmarcher's Cruelty which is a very meh set and I've never seen it used. All in all, necro is a wonderful class, tricky to learn, but once you master the rotation, you can definitely pull incredibly high numbers
Necromancer is by far my favorite class in the game because of it's versatility, it is however lacking in damage in content, it doesn't matter much if you can parse as much as the other classes in a dummy if you still do 20% less damage than the 3 button zoo sorc whenever you're doing actual content. As it is right now necros are only good as buff bots and to clear trash packs.