Why don't self posts gain link karma?

I know that many people take issue with the number of image posts submitted all of the time. But it seems like the reason behind this is pretty obvious - only links gain karma.

Why don't upvotes on text posts count for karma? Both help to boost activity within the Reddit community, so it seems to me that both should count for karma.

I imagine it would also help to even out the number of self posts submitted.