Why are there no down votes for comments as there are for questions and answers. People sometimes use the comments to give answers as well.
Because people should not give answers in the comment section. Such answers should be flagged and deleted, not downvoted.
Comments on Stack Exchange are an afterthought and essentially a "second hand citizen". As you can see when you try to write one, the clarification iterates that they are for asking for clarification and explicitly not for answering. You can not accept a comment as the right answer, and you can not edit mistakes in it. They can also be removed at any time.
This is site wide (through all of SE's networks) and is not going to change as it would affect all sites. This has already been covered here in the meta: