Because GPL is hard to work with. I want to write code, not dig through legalese or discuss with a lawyer whether it's okay for me to do X or Y. More often the easier option is either to completely isolate GPL'ed bits (i.e. run GPL software on a separate machine so that there's no possible interpretation under which we're modifying it), or use something with a different license.

Or are you saying companies should be more willing to GPL their products? That's unlikely to happen.