Anyone who spreads spam inevitably earns a reputation as a spammer. And deprives itself of the opportunity to work successfully in the network in the future. And including earning a reward.
By posting trash, a spammer can get a lot of likes from random participants who have no weight in the community. But the promotion of the post and receiving awards are provided by likes of serious participants, which are relatively few. But which earn a solid weight in the points of the community. Such a participant will not like spam. But he may well put a dislike. And it is their one like or dislike that matters for moving the post up. Or down. Not dozens of random ones.
In addition, spammers are unwittingly remembered. And I don't want to like even interesting posts if I already know their author as a spammer. Or as someone who supports spammers with their actions.