The problem is this, well not a problem, just how the math model is geared. PnG has focussed the main potential in the 500x to 1,500x range on the newer slots. E.g. on Reactoonz you will often see 750x and a few 1,500x for Garga.
Reactoonz max win is 4,350x bet, probability is 1:3.19076E-11 or 1: 319,076,000,000, in short 1:319 billion spins.
BoD 5K probability is 1:50 million spins
Moon Princess 5K probability is 1:160 million
Firefly Frenzy 10K probability is 1:43 million
Many are in the 1:billions range, so probably we are never going to see them.