Are they?
Demons are perfectly capable of controlling the body of their host - it's just that in general they have little needs to control their appearance.
I'd say that in most cases the demon in question has just "arrived" (and thus was still disoriented) or saw no need for hiding (Broken Circle)
And wouldn't in general the demons that are best at hiding be also the most powerful and dangerous ones?
Actually, the only abominations who didn't mutate their hosts have been either willing ones, or could have been willing.
Connor was willing.
Marethari was presumably willing.
Evelina was willing; Walter's testimony shows that she turned into an abomination to kill the templars chasing her, but apparently shifted back into human form afterward and was "ashamed," hence running away and hiding. It's behavior consistent with Connor, and no unwilling abomination has ever thrown off demonic influence.
Uldred was ambiguous, but could easily have been willing, especially since the demon considered itself a continuation of Uldred in a way; most abominations seem to discard their human identities altogether.
And Wilmod was an anomaly, being a demon specifically bound by a mage into a host it wouldn't normally enter.