Comments by other users cannot be deleted or downvoted by users such as me. Why don't we enable the feature of downvoting comments and many downvotes causing deletion? Please don't get me wrong. But I feel that too many useless comments can distract the readers from getting to the actual meat. I know there are good comments but there are also many a times useless comments. I understand that which comment is useful and which one is useless can be subjective but I think there are enough experts in this site. Thanks!
If by "we" you mean the community of users on this site, then we cannot downvote comments because the system's software is designed in that way and we do not have access to that type of changes to the software.
If by "we" you mean something more abstract and general, then the answer is that Stack Exchange made this conscious design decision when Stack Overflow was designed in 2008, and it is extremely unlikely that they will undertake such a radical redesign at this stage; in other words, the barebones answer to "why doesn't the downvote-comments feature exist" is "because Stack Exchange doesn't want to implement it", for a variety of reasons. To explore those, see the ten-year history of the discussion starting at Allow downvoting comments and its plethora of Linked questions on Meta Stack Exchange.