Among other reasons, fundamentalists state:
- You shouldn't make users download more than what's needed
- You shouldn't trust the code of others for your purposes (if you want it done right, do it yourself mentality)
Don't Reinvent The Wheel
Why write code that's already been written (better)? A good programmer is a lazy programmer, so be lazy. The tools are there -- use them.
Do More With Less Code
Save Time -- You Don't Code Your Own OS, Do You?
Chances Are, You Aren't The Expert
Do you have more reasons? Please share them!