In addition to the answers already provided, here are two more reasons:
money and
time.
How much did you pay for the modules you are using, or will use? My guess is "zero". It is my understanding that most (if not all) of CPAN modules are created by volunteers. Perhaps many authors (such as myself) spent nights and weekends away from work writing tests and POD -- without monetary compensation. There is no army of well-paid Micro$soft hackers churning out CPAN modules on the company's dime. Maybe you get what you pay for :)
Do you know of any other free software repository in which most of its library components have ample/accurate/coherent comments? Please provide some links.
most of the CPAN/core modules lacks comments
That is an amazing claim, considering there are more than 10,000 modules on CPAN.
