It’s because the real underlying asset is Vests, not Steem Power.

I did a post about this in August:
https://www.palnet.io/steemleo/@preparedwombat/3vs4h1-the-steem-power-you-earn-for-holding-steem-power-revised

It has links to earlier an earlier post where I also discuss the implications for Muslims and a @lukestokes post emphasizing that while on the surface it may look like interest, it really isn’t. Your real holdings are Vests.