I intentionally used "make profit" to make it easier, but actually it's compensate loss from inflation. SP holders who don't curate are losing about 8% annually, but if they curate, it will decrease to around 4~6%.
Meanwhile, STEEM holders are losing 9.5% a year, since they don't get 15% of inflation reward. Merely removing curation reward keeps the difference (15%) same, but if we change curation reward to inflation reward, the difference will become around 33%.

Details can differ, but I think the core idea is the same, as I mentioned in my last sentence. Voting is not free lunch.