Its much harder o write a nice looking application than just write a standard one.
Most programmes also are not very good designers (including myself).
Either your skills are on programming or designing. There are only a few people who are good in both.
Compared to commercial projects, open source projects are often run by only one person which gives them a big disadvantage.

If course I can (and do) have a look on iPhone apps and try to adapt parts of it, but without the right skills (incl. GUI programming wise) its challenging.