Some companies might be in the position to make that choice, but I'd guess most aren't really equipped for it. First of all there's the issue of getting your changes upstream, you have to convince the community it's worth their time to help maintain your code, plus a quality bar you have to meet. And then once it's upstream you're committed to drinking from the firehose, instead of updating at your own pace, and there's extra scrutiny on every change you make.

I really think most of these proprietary users are just figuring things out as they go, and doing that upstream 1. wouldn't be accepted, and 2. wouldn't make sense for them in the first place. But I'll admit that's speculation, I don't have intimate knowledge of what everyone's doing :)