You must just be having terrible luck, because I have had each one of the jackpot starters pop up in easily less than 3 hours when i most recently did it.
And the point of them being so rare is because they aren't standard starters, if you really want to get one you'll try to.
In the case of Eevee, it is probably one of the best starters for the early game, because you can easily get an Espeon or Sylveon before the first gym (same with Riolu really), which is another reason why the chances are so rare.