Except that entries to those contests have value that isn't tied directly to the contest-giver-- android apps can be resold independently, and netflixprize entrants are developing machine learning algorithms that have tremendous value (and it's open-source, IIRC, which is a whole different kettle of fish).
In a logo design competition, the elements cannot (easily) be resold or repurposed, since they are specific to the specs of a particular client.
Also, winning one the Android or Netfliz prizes confers lots of value in terms of publicity, which can't be said of 10,000 small logo competitions.
One other note: Rentacoder, for instance, has specific rules prohibiting posting design comps or source code to prevent uncompensated work.
In a logo design competition, the elements cannot (easily) be resold or repurposed, since they are specific to the specs of a particular client.
Also, winning one the Android or Netfliz prizes confers lots of value in terms of publicity, which can't be said of 10,000 small logo competitions.
One other note: Rentacoder, for instance, has specific rules prohibiting posting design comps or source code to prevent uncompensated work.