It's not about reading ability, it's about the way things are written. There's been plenty of research done on how to write notices and warnings etc. that will catch attention, and how to persuade readers to follow the directions. But of course "user interfaces" are often not "designed" by designers at all, much less anyone who's ever studied the research. (Not that most designers study the research, either, but they're more likely to than Joe Programmer.)
Example: Jakob Nielsen publishes research that shows "people on the web don't read." The sample content used to determine this? The list of tourist attractions in North Dakota.
Garbage in, garbage out.
Finally, the vast majority of text on the web around forms etc. is useless, poorly written, obtuse, abstruse. People have been indirectly trained to ignore it. It's not surprising that most users ignore that messages you took the effort to write.