In retrospect, I think the problem is pretty clear: Tumblr, like most social networks, values creators as a means to an end—the people writing and posting make the platform more valuable and more worthy of the average person’s time, but the rewards for that value quite often go to the people who operate the platform, not the people who make it relevant.
Ernie Smith on value.
See also Facebook, Twitter, Amazon, Steam, et cetera, et cetera. Basically any and every content aggregator.
Incidentally, the post this quote is taken from is about urging people to start an independent blog which, obviously, is something I am always here for.