I think it's because they write mostly for themselves and mostly as fast as they were thinking of things. It's better to have to interpret something for your publicist, then to lose a thought you had while focusing on writing.
There's some notable exceptions by the way, you can't have a post on academic handwriting without mentioning Dijkstra:
http://www.cs.utexas.edu/users/EWD/ewd13xx/EWD1300.PDF
Funny, I hadn't recently read the article I linked before posting it, and in the first paragraph he actually explains why he writes so prettily. Although the context is programming notation:
"[..] the underlying idea of which is that, even if you have only 60 readers, it pays to spend an hour if by doing so you can save your average reader a minute."