Because we are not just straight proof-of-stake.
The reward pool has a network and, if there is no sort of value being added to the network, it makes the rationale to participate much less compelling.
When I say value, I mean knowledge, art, music, humor etc etc.
The "stake and take" mentality is a slap in the face of every one of the users that strive to do the above.
It cheapens their labor will the lazy stack coins... Coins that will degrade in value as inflation continues while the pool of those willing to buy those coins diminishes.
I used this same analogy on another comment.
Would a moth be attracted to the lamp lest there be a light? Be the light.