Typically in an open house just anybody can walk in, can’t they? It’s not like they ask you for ID.
And we took quite a while to figure out what we really wanted (and we were comfortable living in the place we were in, so we didn't need to make a change...)
We only ever made three offers, two of them on places we saw on the same day.
Obviously that doesn’t mean that one can’t pretend well enough once or twice, but not at a scale where this becomes a viable avenue to criminals.
Seriously, go visit one and see what it's like. It's not the CIA Headquarters, it's usually a minimums wage newbie agent squatting in a house advertising for clients.