What if those bigcorps don't merge some improvements for getting an edge over the competitors? And, what if every one of those bigcorps do the same? How would that impact the project in the future?
They pay the costs of maintaining their fork. Thus there will be constant internal chatter of is the non-merged stuff really valuable enough considering the cost of maintenance. Sometimes yes, but sometimes no as well. So overall that is a long term win.
Already the case for Apple LLVM which has optimizations for Apple Silicon not upstreamed and never will be upstreamed.