For example, one guy specialize and dedicate his life to metalworking. If I try to know everything in the world, I could barely scratch the surface of what that guy learned.
People can even specialize in multidisciplines. They don't know as deeply as a specialist in a subject area but they know two area well and combine them into useful combination.
It would seems that today's problem is more about the limit of human beings' ability to store and synthesize information across vast disparate field. In other words, we generated so much knowledge, but so much is just kept there not being used.
Spaced repetition is a good example. It's a very powerful memorization technique, but it is not being used in colleges and schools, except those who discover it on accident.
It's also a timesaving tool in the programming profession. Instead of googling and wasting 5 minutes for answers to our programming problem, we can save 5 minutes for many common tasks we memorize. It won't save us from debugging woes but at least we get to the important programming problems faster.
There's also a storehouse of reliable information on self improvement written by psychologists who done experiment and research things like willpower and discipline, why it fails, and so on. Instead, we got distracted by techniques that seems to work but have no scientific basis, or we get distracted by self improvement gurus that have no idea what we're talking about.
We are specialists but we miss a ton of useful stuff that would be useful to our specialization. It's like missing a thousand useful book every year because you can never read them fast enough.