I hope you are aware of, that you do not have to give Steemit Inc your phone number. There are alternatives (which can be well appreciated by some people). You can create an account on: https://anon.steem.network/

Why there is a cost of account creation is well described in this article:
http://bytemaster.github.io/article/2016/02/10/How-to-build-a-decentralized-application-without-fees/

Hive account@dantheman wrote it even before Steem and Steemit existed - there is no other blockchain which has no fees!