Some of them don't even
meet at IETF meetings, because they see no value in presenting their
work-in-progress to the masses.
Some of us believe that is an arrogant approach that leads to
work coming out of a WG that has not been exposed to IETF-wide
review soon enough, and is therefore at risk of either having
a very hard time during Last Call and IESG review, or of ending
up as a minority solution at best.
It's a fact that humans are driven by deadlines, which is one
reason why interim meetings add value, and there are some topics
that can indeed succeed without meeting at IETF weeks, but for
most topics, IMHO we really need the exposure to the IETF masses.
Brian