Out of generosity would be a good reason. Though for megawhales like Steemit Inc employees, it'd also be in their best interest to diversify curation. That'll lead to better retention, a better trending page, user growth (assuming marketing is successful) and ultimately a higher price.
Of course, the delegatee is automatically a proxy curator.
Some whales may want to cut a deal where the delegatee/curator sends back a cut of curation rewards as liquid Steem over the long term.
Whales may also delegate SP to their sockpuppets. That said, this would all be visible on the blockchain, and abuse can be fought easily, so the preferred route for that is still powering up sockpuppet accounts individually.
Has this changed? Can I receive curation rewards if I delegate to someone now?
Where do I find whales who are ready to cut deal with curators? I have asked couple users but they are not interested, meh :(
#Sockpuppetaccounts