Side note: as I was building this, I discovered that Google (the owner of the .app registry) seems to have banned registration of a bunch of interesting domains. This includes most swear words, street drugs, etc. Also on the list: gene.app, bacteria.app, pigeon.app, lens.app, and glove.app. Google Glove anyone? :)
Some I checked were already taken and others were "premium" domains.
Most of the shorter ones are "premium", unfortunately, that's just the way Google has priced them. I was able to get minus.app for about $60, though, roughly the same as a "non-premium" .io domain.
Which domain were you trying? About 30 have already been registered since I posted this, but it takes about 10 minutes for them to disappear from the list.