They use whichever means available for them.
Same as any "rational" entity does.
You call it exploitation.
Some of what you call 'rational' is actually psychopathic in many instances - in fact, totally criminal - however, they use their 'whale' status to control the courts and government where necessary. This has been proven over and over again.
Is it any more exploitative than paying to some of these bots and enjoying some nice ROI?
Absolutely, yes it is. The former involves deliberately causing imbalance an in order to exploit it and limit the reach of others - while the latter involves simply using whatever methods are available to attempt to resolve the situation (Which will not work because it actually feeds the cause of the problem). To think that the two are the same is like thinking that a fisherman and a fish are the same in their level of exploitation.
How did the whales become whales then?
I don't have full data on that, some were developers at Steemit as I understand it - who presumably mined. Some joined later as I understand it who were not part of Steemit, but only once a large amount had already been mined.