First you should read FAQ section in steemit.com (it is for all users).

Creating account on steem blockchain needs some steem as delegation. which was about ~27sp (recently reduced by witnesses to ~14 SP)
@steem creates accounts on steem blockchain for free! to preventing abuses it needs your phone and email. (email is needed for recovering hacked accounts)
@steem should undelegate new registered users to create new users! and it is important to our community.
without @steem, you shoud pay about 25 USD to create new accounts. but can you do this for all users?
@steem is controlled by the founders of steemit and steem blockchain.

(developers of other coins like ETH, BTC, ... are holding at least 40% of that coin. you can't say it is decentralized or not!)