We do tell them not to go in.
Its usually a head in cockpit syndrome not aviating navigating communicating. They get focused on there charts and either drift up because they are flying with out looking or ATC give them an instruction and there head goes bang and they have a complete common sense failure. Its more a failing of not being able to do 2-3 things at once. Their handeling hasn't got to the stage that their hands and feet can look after themselves ( abit like changing gear in a car)
Or what was normaly the case with my students the local mil unit decided to put them on a vector in class G airspace or a not below instruction and the silly sods excepted it. And when it was obvious that they were going to go IMC carried on anyway.
MJ