Vast majority of users are not capable of recognizing that such comments are spam. They are completely clueless and/or don't do basic due diligence by checking someone's blog. You realize that when you see them responding to these spam comments thinking that they are honest and crafted just for them showing real interest in what they post.
These spam comments are manufactared to fish for potential votes from such oblivious users.

The problem is that spam posts simply spam the Blockchain.
More importantly, it drowns the visibility of posts created by decent content creators.
Would you like your post that you put effort in creating quickly disappearing and becoming invisible because of it being pushed away and drowned between mass of spam posts?

Sadly, majority of posts published each day are some sort of abuse like plagiarism, copypasta and/or spam attempting to farm/fish on curated and/or popular tags.

Also, these posts seldom have "decline payout" option selected, meaning that they are attempt to fish for random vote.

How does it bring value to Steemit and recognition from outside, if any new outsider who tries to check up what Steemit is about just see trash, spam and exploitation posts everywhere?