And that lack of royalty is part of why the OGL failed to stimulate cross-pollination: the majority of the best stuff gets held back from being open content because to do otherwise devalues the original by "the death of 1000 cuts."
Meanwhile, the stuff that IS open content is, as you stated, spread out among numerous products, many of which are hard to find. That, coupled with other aspects of the OGL means that using someone's open content often means the purchaser must track down an additional supplement- which may be impossible, depending upon the rarity of the sourcebook.