Why do national laboratory presentations contain so much info per slide?
Let me try to answer for the tendency in my own discipline (particle physics) where we have this problem across the board (i.e. universities too). 

Much like questions and answers on a Stack Exchange site, those slides are expected to form a resource for future investigators. We know there is too much there for anyone to absorb in the meeting, but we also know that more people will dig these slides out of the archive over the next year and study them then actually attended the meeting in the first place.

Yes, in an ideal world there would be a technical report and a deck of slides, but in fact there are only the slides.



Personally I try pretty hard not to do this, and the result is a lot of backup slides and a lot of little URLs hanging around the bottom of the slides.



As an aside, I think that PowerPoint and similar polished slideware makes stuffing them (over-)full way too easy. I use a LaTeX base for mine (just the old slides class with my own library of macros for a long time, but I've started using Beamer) and these tools encourage a better style. 

Alas, I know all the tricks to squeeze on just one more thing.