Can they? Yes. But the best way to hide scares (drop doors, sliding doors, and scrims) introduce mechanical complications and a separation between the guests and the performer that weakens the scare's impact. Also, BECAUSE of their effectiveness, they can actually desensitize guests quicker than a full-size boohole.
Hiding scares is actually pretty difficult (and gets progressively more so the further into the run). The scare has to be visible to be effective (scares from behind rarely work), and has to fit a person which makes the holes noticeably large. A lot of scares hidden by curtains are impossible to see during rehearsal nights, but once it starts moving a gap forms.
It's a big reason why distractions are so important. You will rarely see a single performer in a room by themselves, unless there is a big set-piece. More mundane rooms will always have 2-4 scares. Because those other scares are actually distractions in and of themselves.
This doesn't take into account guests experience. Someone who has been through a lot of houses knows what to look for, and looks for clues whether they mean to or not. Less experience guests (or people who can allow themselves be overwhelmed by everything) don't look for the clues, and can get bigger scares.