My fairly crude theory is that those sites have slightly broader and thus less tech-savvy userbases, so searches might represent ingrained ways of accessing the site more than they would for Reddit or Tumblr.
As even the least-tech-savvy becomes more aware of faster ways of accessing the websites, and browser software starts to pre-empt user intention more and more, this kind of convoluted search starts to drop off.
Not sure if that theory holds water, or is in any way necessary.
