Given enough time, people forget. Besides, the people today need
  the fame. The people 50-60-70-100 years ago don't need the fame
  anymore. They're dead. So new people get the fame. I just roll
  my eyes a bit when I notice it. "Oh look, that's just a
  reworking [x]'s idea... oh wait, it's not even reworked, just
  reworded". "Oh look, someone dragged that old theory from the
  dustbin and stuck their name on it. Groan". To me, makes Science
  look bad when that happens. But, as most don't seem to notice or
  mind, I guess it's just how it goes. I notice though and I like
  point it out when I see it. They got their fame off of somebody
  else. I feel justified in pointing a finger and going, "Bah,
  thief". Maybe they didn't steal and just happened upon things
  the same way. I mean anybody who reads a lot is likely to
  accidentally pick up combinations of words or proofs or ideas
  and not _realize_ where they came from. Still, it's good to know
  sources. I enjoy tracing back sources.