Why spam comments when there is a directory?

For companies spamming out there trying to get backlinks instead of leaving comments I then have to delete (reason for the upgraded security on the comments) just go to the links directory and add it into that its what its designed for. At the same time the links need to be of value e.g. the marble polishing and modular home companies out of Florida have little relevance to anyone here in the Philippines. I did initially delete and block emails which weren’t getting caught by filters but it seems the most recent ones arriving are manually written which is why I am just going to filter all comments in future so that I can track and remove any.