You've asked a very relevant question. Many of the large accounts you see here were beneficiaries of the Ninja mining that allocated large Steem power to the aristocratic few that were the early settlers on this space.
Also, a fraction of the whales bought their SP when the price of Steem was trading at about 80 cent and then went on to earn massive Steem during the last bull run.
Lastly, there are a few accounts that got large amount of SP as a gift from their ninja friends.
By the way, the events above are now history. Steem has never been this cheap. So, instead of complaining, we should put some money into Steem and make some huge progress.
It was early adopters advantage. The earning pool back then was very high and steem was also so cheap, they earned easily and some bought large quantity
A majority from my understanding mined it from the so called Ninja mine. They never did an ICO like other coins and mined it on the quiet or very few were able to from my understanding.
There are some accounts though that weren't part of this scheme and were some of the first accounts on here. Steem was easier to earn then and posts were in the 1000's of Steem so it was easy to accumulate. Imagine getting a $100 post when Steem was at 7 c, you would be looking at 1000 Steem per post which is crazy.
Other accounts invested and bought in when the price was low and have been adding to the account every time the coin drops in value. Just by doing a little posting and commenting daily if you have a large stake you will grow quickly.
I think it is possible to achieve 50-100 000 Steem if you stay busy for the next 5 or 6 years powering everyhting up.
Some bought it (preferably on a dip) some stole it in the ninja mine. (google it)
If I had to guess, I would say that the majority of people probably purchased their Steem when the price was really low. There are some that have been here a long time and they have benefited from whales upvoting them. There are also others that have taken part in some of the unsavory practices the other people mentioned. I know I would not have as much SP as I do today if I had not purhcased some STEEM when the prices was favorable.