The use of the term goes back to bitcoin. I suspect the idea was to answer the question "where does the money come from" and mining seemed like an OK analogy... ie they were finding these btc the same way miners found gold.

Of course, this is not true. The new BTC (and steem for that matter) are created out of thin air.

In essence, crypto "mining" is just a race between a bunch of computers to solve what amounts to a very difficult guessing game puzzle. The winner of the race gets a bunch of coins, and also is tasked with writing the new block and recording all the transactions.