Millymollymoo - Thing is there are 1000s of parents turning up - literally.
Take, for instance, Tiffin School in these parts. About 1,600 boys sat the exam this year. All of those had parents, or some interested adult. It requires an effort to sit the exam (you opt to take it), so you can assume a level of interest on the carers' parts, which will extend to viewing the school.
Moreover, you can assume that there were a few who looked at the school and decided not to enter the exam AND some, very keen, parents who will scout the schools the year before, make a short-list of schools they're keen on, and then return the next year (yr 6) to the short-list with their dc.
Seriously, I met lots of parents doing that.
So there just aren't the hours in the school day to do one-to-one with all those parents.
Not to mention the disruption to primary schools of all those dc taking time out of the primary school class to go visit secondaries in school time.
Not saying you shouldn't do it; if you can, it is undoubtedly a great way of getting an insight.
Just explaining why schools run an Open Evening/Day and that it's not quite the same (ergonomically) as showing around recruitment agencies.