I'd pay $500 for $1000 worth of SP.

Not that I don't value SP. But because liquidating SP is far more valuable to the seller than buying SP from them is to me.

I can go to the market, buy STEEM and power up.

The seller will have to wait two years to liquid $1000 worth of SP.

They will have to wait one year to get the $500 I'm offering. They will need to go through the rigmarole of trading out each week for a year. As well as run the risk of price fluctuations.

Also there are a lot of people on Steemit that just write blogs and are motivated by cash in their bank account. They don't curate. They have no desire to trade out every week. Where I think "I can grow this money and the price could go up." They think "the price could go down and it's a bit of a gamble." I know this because I've had this conversation with a few people.

If I were to offer $5k cash for $10k worth of SP they'd snap my fingers off! "Cash now" is a very powerful incentive.