Because last I checked, Microsoft, Apple, Oracle and Google still are the biggest companies and their wealth rests primarily on closed source software.
I would think for the "largest" transfer of wealth, we would be able to pinpoint some poor exploited geeks coding software juxtaposed against some rich fat cats making money off of it.
But Linus Torvalds doesn't seem poor and IBM/Red Hat, while rich, is much smaller than Microsoft.
Apple I think relies heavily on the BSD project (I think they might be even using the same kernel?) and Google on Linux. There's also probably a lot of open-source software they use behind the scenes or which aren't as big.
In Linus' defense, I would probably pay the person that wrote and maintains the software that literally runs the world pretty well too. Can't afford not to.