Difficulty of having profound scientific discussions with more than 2 or 3 people?
It seems to be related to a kind of "Peer pressure".

This is due the change of the context. A profound discussion is regarded as confidential talk and you will notice that the voice volume is lower than a discussion with many people.

But why is a profound discussion regarded as confidential?

Maybe because you could touch scientific taboos. Every time you touch the foundation of a world view you will confront all participants with their fears. The world view represents a aspect of security. Exposing lacks in a world view will expose the fears of them too. Then it's much more difficult to find a common ground for a solution. Not everybody is willed to accept the implication of a certain insight.

A discussion with more people will tend to go towards superficial topics, cars, home, family, weather and so on.

But it depends on who initiate the talk and if this person is keen enough to enforce this topic. And I think that Keegan is right, sometimes we just want to be polite and we want the discussion to be more open ... maybe more open to the "general context" of the situation itself.