If you play them to the best of your ability.
DPS is only perceived as less difficult because you can get away with less than your best effort. You can auto-attack, hit a few abilities, and most people won't notice how awful you're doing. Yeah, that's pretty easy. Sometimes you'll get someone who is paying attention to meters and stuff and you'll get kicked.
Tanking and Healing have minimum thresholds. You have to at least exhibit a certain amount of skill, otherwise you just can't complete the dungeon. It's perceived as more difficult because if you fall behind this curve, you wipe.
So yes, being a lazy DPS is technically easier than being an average/good tank or healer. But why would you want to be a lazy DPS? I like to do the best I can, push for the best numbers, try to hit those interrupts, and avoid taking damage. Doing well is fun for me, but it also makes DPS a lot more difficult.
If I could get instant queues and a satchel with my Ret paladin, I'd still pick Prot. Getting good DPS as Ret is much harder than tanking as Prot, hands down.
TLDR: Tanking/healing is only perceived as more difficult because if you mess up, you wipe. Not because it's harder to avoid messing up, it's just less forgiving if you do.