The method that these exchanges you mentioned implement is to change the STEEM to BTC or ETH. They do this daily.
They do not accumulate STEEM.

As long as streem has value, they will keep buying and selling.

At the moment you can see how the STEEM value is increasing. This is motivated by several reasons, among them the great publicity that Steem Blockchain is having with all this conflict, in addition many people are increasing their holding to take advantage of the next snapshot.

But soon the steem debacle will come. So let's pray that HIVE is consolidated by that time.