You're not forced to power-up the Steem, so the comparison to Bitcoin is like comparing apples with oranges. You can hodl Steem in your wallet, same as Bitcoin. Which is why we need a dynamic range of stake-duration with increasing incentives. Those who want, can stake it for multiple months or even years and be rewarded with some incentive in form of APR, while you can have the luxury of unstacking it within a shorter period of time, but with less APR.