RE: Why Steem offers one of the most secure wallets in Crypto
If someone else has a copy of them or you are storing them on another site such as exchanges then they are never truly yours as they can be stolen or hacked at any time. This is why many only "hodl" their coins and don't trade with them on a daily basis - this is also a big reason for the ridiculous volatility we have been seeing in prices as only a minority trade actively with their coins.
Actually the vast majority of Bitcoin and other crypto users store their coins on exchanges (even if they're not actively trading), and this is a massive problem.
This is a conclusion of basic math (historic number of transactions vs. exchange accounts as well as amounts held in known exchange addresses).
I would also note that account recovery applies even if you weren't registered via @steem. For example I onboard people using steeminvite.com. If you look at the accounts I created such as https://steemd.com/@one-person, they have 'demotruk' registered as recovery account. Thus if @one-person ever got hacked, they could contact me, I could investigate it and help them recover if I am confident they are the real deal.