Well, yes, it is about extending the knowledge of humanity- but it doesn't happen in a vacuum, and is subject to a lot of the same constraints as any other human activity. And, as you say, there are incentives at work- if the john_b's advisor had written "release usable MATLAB toolkit for $doing_whatever_john_b's_thesis_does" into his grant as a deliverable, you can bet that both john_b and the advisor would have made sure that it was in a releasable state, and also that the advisor would have had funds available to pay john_b to clean it up and get it ready to go- they would have been specifically allocated for that purpose in the grant's budget.