A lot of these major whales are simply early investors who made some excellent calls to put themselves in the position they're in now. If they're investing in other things too, they maybe don't have the time or desire to post, but it stands to reason that they would reward people who are positively contributing to this community. After all, their investments will continue to grow as STEEM grows. By doing their part to reward good posters, they're keeping the reward pool circulating to the right kind of people.