Why is link karma typically valued more than comment karma?

Without links, Reddit is just a forum.

Functioning as a link aggregator allows Reddit to call itself the "front page of the internet."

It's effectively crowd-sourcing the process of stumbling on an interesting website/picture.

Because it's easier to get link karma by posting a shitty meme than it is to get comment karma for posting a shitty pun.

Because link karma is more visible. It hovers over your front page. To see comment karma, you have to look at your profile our whatever.