Power

Did you hear the one about the guy who used greeting cards to systematically take over the world and brainwash powerful global leaders and entire parliaments?

No. You didn't. Because the only powers greeting card writers have are an uncanny ability to make the eyes of already sad, in-need-of-well-wishes people brim with tears. They also occasionally have a groan-worthy way with puns, and the luxury of wearing pajamas to "work."

 
"What's happening to me? Why are my eyes wet? Did I just read a really sad and effective greeting card, triggering deeply felt and bittersweet emotions?" (Source)

At this point in the game, it's the publishers who have the most power. You can toil away all day, sharpening your pun skills, crafting the perfect couplet, but ultimately, card publishers have the final say with regard to your writing. You can certainly self-publish, but be prepared to sell a small handful every now-and-again at your city's weekend craft fair.