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How come HIVE still has 4 years worth of contents that were posted on STEEM, as well as all the accounts that were created on STEEM?

The first 41M blocks of your chain, are coming from the STEEM blockchain. Your blockchain exists because STEEM was there before. Without STEEM, Hive wouldn't be there.

So how can it be 'created out of nothing'?

If everyone in a room has a blue rock, and I give half of the people in the room a red rock, I did not steal a red rock from the other half.

Airdrops often have qualifications to get them, Sign up for this, like this, register here. Hive had qualifications too.

Nonsense, it was created with the Steem Blockchain and every single person who had a stake on that blockchain . lol
And accounts were plundered for the purpose of enriching themselves minimizing their need to make any financial investment.