They already do things like that... And those things are good for people who want to spend new money, to build towards future savings.
But it does nothing for people who have amassed licenses for games they no longer want.
By getting 10% back for old licenses, consumers would have the opportunity to reinvest money that they've already spent. with a large enough collection of unwanted games, they might not even need to do any additional spending to get something new.
That's great for people who have games in the collection that they no longer value.
There's no reason that credits for new purchases and buyback of old purchases can't co-exist, but the two concepts would cater to people in different situations.