Because they are not writing for non-scientists.
On a side note, I'd say that many researchers don't do a good job of conveying their ideas clearly (it gets worse with conference presentations). It won't really matter in what order you try to read their papers.
[1] http://blizzard.cs.uwaterloo.ca/keshav/home/Papers/data/07/p...
Look, I get that there's some natural professional context and lingo that goes into these things, but for all the angst that goes into what esteem that population at large holds up the science community
making their work more accessible to both novices and interested outsiders would be a nice step in the right direction
This seems like a good approach in my humble opinion.