It's rare that this weighs heavily compared to the administrative burden of complying with the GPL. Most open-source code (especially things like coreutils) is effectively commons: something which many companies rely on but doesn't represent a competitive edge to them. So there's already a strong incentive to contribute upstream: it's easier than trying to maintain your own fork. A competitor hoarding their own contributions is more likely to be shooting themselves in the foot than giving themselves an edge.