Because there will only ever be 3 major multi-billion tech companies that stand to benefit most from OSS.
Because they own their markets, where they can wait to find out what OSS software has become successful and either just profit from it by hosting it directly or invest the paid resources into replicating it - an opportunity and reward not available to anyone else that doesn't have a large cloud market share.
Because they're using their profits (made possible from hosting other's OSS investments) to finance their own OSS investments which will have infinite paid and support resources that Indie OSS developers will be unable to compete against, eventually leading to building a brand where there'll be customers that will only use OSS by the cloud vendors first, since they're commercially backed and dependable.