Here's why:
* Macro Cosmos' effect: Macro Cosmos prevents monsters from being Special Summoned from the Graveyard. It has no effect on cards being discarded or sent to the Graveyard.
* Discarding and Banishing: Discarding a card means sending it from your hand to the Graveyard. Banishing means sending a card to the Banished Zone. These are two separate actions.
Therefore, Macro Cosmos only prevents Special Summoning from the Graveyard, not banishing discarded cards.