Because everyone is locked in for 2 years

Its 3 months.

You can hold the Steem Dollars, which keeps the money in the network and pays you 10% interest

There is currently no interest for holding steem dollars.

You can convert the Steem Dollars to Steem and sell it immediately on the open market for real money

Steem and Steem Dollars are real money, however you can convert both for Bitcoin or Fiat Currencies on exchanges. There is no need to convert Steem Dollars to Steem First, they can be sold directly as well.