Because I can capture 70% of your experience in your code -- in your library -- and build directly on top of it. If you use my library, you have not capture

Because I can capture 70% of your experience in your code -- in your library -- and build directly on top of it. If you use my library, you have not captured my experience to write said library. If you need to write any higher level library of your own, the most problems you will face are exactly the ones I already solved once. I could write that library faster and more correctly than you. (the words 'you' and 'I' do not (necessarily) denote you and me)