I'll give you the benefit of the doubt for the sake of argument. Systems aren't appealing because the ROI stinks. Even if the GUT was winning, it would not even come close to beating the ROI that a solid VB player could get. I'd love to see the GUT out perform a VB or Bias play only betting 3 numbers per spin. Whether you are betting 1 number or 5 numbers, the AP will still have a higher ROI and likely is far fewer spins. Pretty sure you are going to say you need to cover more than 3 numbers for it to work, so, lets say you needed to bet 12 or 15 numbers or whatever number you decide, the ROI would still be crap and the volatility would be through the roof. Betting 34 numbers has a higher hit rate than any other strategy, but the ROI is still crap because it will lose in the long term. All the GUT is doing if cutting it in half and cherry picking the numbers to bet based on previous results. That still hasn't solved the ROI problem, let alone the house edge.
None of us are willing to waste our time with a system that will earn us fewer units and have higher volatility than a very basic VB play. It doesn't make sense to do something with 15 units that you could very easily do with 5 units. The other big difference is we can measure our edge and progress and know when we are winning and losing according to our edge. Systems can't do this. Instead, you just have to buckle down and have faith that your system will come through with not much else to go on but your faith in the system. AP is a documented and measurable science. Systems are the cults of the religion world of gambling.