Quote from the white paper:

Because Steem wants to encourage long-term growth, it is hardwired to allocate 9 STEEM to Steem Power (SP) stakeholders for every 1 STEEM it creates to fund growth through contribution incentives. Over time this drives the ratio of the total STEEM value of Steem Power balances to the total of STEEM balances toward 9:1 . (It seems likely that the ratio will be somewhat greater than 9:1 due to continued net Powering Up of the newly printed STEEM.) It also means that long-term holders are almost completely protected from the dilution used to fund growth.

What this says to me, everytime a new Steem is created for content, 9 more Steems are created and distributed proportionally to SP holders. That's where the interest comes from.