Why there's no 'edit_comment' capability?

‘edit_comment’ is a virtual meta capability which finally is converted to ‘edit_post’ related one, like ‘edit_posts’ or ‘edit_others_posts’. Look at wp-includes/capabilities map_meta_cap() function:

case 'edit_comment':
		$comment = get_comment( $args[0] );
		if ( ! $comment ) {
			$caps[] = 'do_not_allow';
			break;
		}

		$post = get_post( $comment->comment_post_ID );

		/*
		 * If the post doesn't exist, we have an orphaned comment.
		 * Fall back to the edit_posts capability, instead.
		 */
		if ( $post ) {
			$caps = map_meta_cap( 'edit_post', $user_id, $post->ID );
		} else {
			$caps = map_meta_cap( 'edit_posts', $user_id );
		}
		break;