Reinventing the Wheel
I prefer to write code from the ground up when possible, rather than using unknown/untrusted outsider code.
Yes, it's often reinventing the wheel, but I know exactly what's going on. I could audit 3rd party code before use - but we all know what it's like picking apart other coders' work . So the time spent auditing is put into home-grown coding.
It also gets around any licensing issues.
Granted, I'm lucky that I don't (usually) have time constraints.
 I recall my CS lecturer telling us a story of his first job, auditing assembler code. He got to a tricky bit that he couldn't fathom, so looked across to the original coder's comment which read: I get lost here too!