Because writing their own takes lots of time and effort. if Blender is suitable for the task, they’ve already got what they need right there.
For another, wouldn’t it sadden you to know that there is a studio that has a version of blender which can do complete cloth simulation, with fully simulated hair, and liquid dynamics, and it has a muscle system, and you will never see those features? (I mean done by them, for I’m sure sooner or rather later the developers will get to these )
It wouldn’t sadden me anyway - the community would certainly not have lost anything, and development would continue as usual. But as well, it would be good that other people are finding value in Blender, and that we can help them get a job done. Though I think that example is a bit far-fetched. It would be far cheaper for a studio to buy a copy of Maya or XSI, than it would be to pay programmers to add those features to Blender.
orion119net: As long as they don’t distribute anything (all work is kept in-house), a studio would be perfectly within their legal rights to modify GPL software and not share it.