There have been dozens of times where I thought "surely someone must have done this before" and then wasted quite a bit of time hunting it down. In some cases I simply couldn't find it (search engines yield irrelevant hits most of the time) and in the cases where I did I still had to spend a great deal of time looking through unfamiliar code with a fine comb to see if it wasn't leaky and bug-ridden.
In a lot of those cases I found that the code wouldn't simply work as-is and needed adjusting, in others it appeared to have been written by someone who didn't know what he or she was doing.
Reusing code is only a timesaver when you can plug it in and not have to worry about it which is why I'd personally rather write it myself than be stuck with code I don't fully trust.
Obviously this doesn't apply when you're dealing with reusable components of several thousands of lines of code.