Well the first thing I would say it that, if we break down the most notable vengeance demons and their traits:
So your point about Jonathan and Andrew doesn't really fit any of those criteria, especially Anyanka. I'm not saying that they can only cast wishes from those select people, but it's "their thing" I guess you could say.
Andrew and Jonathan didn't feel vengeance, they were scared for their lives. True, Warren was a friend, more to Andrew than Jonathan, but they were still more concerned about themselves than trying to do anything to Willow. Andrew makes one comment about summing a demon to "kill that Wica bitch". Both their primary motivations are to get away. Both Andrew and Jonathan say multiple times that they weren't involved and it has nothing to do for them and it's not worth them dying over. They want to remove themselves from the situation, not get more involved in it.
Secondly, this is kind of the same "plot hole" conversation you could have about cell phones. I mean, how many times in Buffy could a situation have been rectified so easily if just two people had a cell phone? Often. We have to remember that there needs to be a story. For example, Buffy in 'The Harvest' clears that really tall fence with super ease at SHS. But there are times when a fence either a similar size, or smaller, that she can't clear at all. And there are loads of other supernatural inconsistencies on the show.
Thirdly, vengeance demons can't cast spells from their own wishes. Now, even if you got Buffy, Xander, Andrew or Jonathan to cast a wish saying for Willow to "calm down", that's probably going to conflict with her own desires. I mean she probably wants Willow to calm down as well (deep down) no? And would that create an alternative timeline/universe?
They care about Willow. Saying for Willow to "calm down" could do anything. Wishes are anything put predictable in their outcome. Vengeance demons take the exact wording of a wish. What generally happens after the wish is bad. The old "be careful what you wish for" incarnation.
TLDR; There are so so many of these types of things you could make argument for in Buffy. There are some decent Buffyverse reasons as to why it wouldn't, but really? It's because the story needs to go on.