Well it is easy...
but it takes up time which people don't seem to like to waste those 10 minutes and it would make more sense to simply back one level lower if that person decides e.g. I am not going to go solo Wildy Wyrm because it's in the deep wilderness and now I am stuck killing 33 moss giants... oh wow look i got "kill 55 red dragons" instead of wasting time on killing idk skeletons. Which instead now... I have to go back and see if I am lucky enough to get a task that will not only give me a good amount of slayer exp but wil also not risk losing my GA (as it is null to have in the wilderness..) And... this applies more to players that barely can afford their set, has high combat skills, and has no "fall back".(Which happens more often than not, because of "cleaning"-but that's another argument in itself... Luck v. Work) Dragons > Chaos Elemental...
Aside from the fact that not many people want to make the travesty to go around and take up their time killing a monster that gives ~200 exp or less per kill and the slayer task giving about 3k of exp total afterwards.
This applies mostly to when people constantly try to get a "easy"(/they know how to kill the monster better than the rest..) and do this process about 10 times and then give up trying. (And no, this is not just my "feels" but it is more reasonable to choose a slightly easier task and not go with super-easy.)
Again, to me it just makes sense to get a choice which difficulty you could downgrade to, but oh well... at least i know now that lower levels have a chance to progress without being complete idiots in the matter. :3 kk, thanks.