Because 80kg less weight is a f**king huge amount to begin with & there's got to be some serious thought & engineering gone into it under the surface to get to that point, Lamborghini used to do a Gallardo that weighed 100kg less & they charged an extra £30k for that because of the effort involved with getting rid of that much weight

Put a 172 and a 172 Cup side by side, look at the difference in weight and consider the cars look pretty much identical and you have to ask what's going on under the surface to achieve that?
No A/C or ABS is only going to be in the region of 10-15kg difference at most, 2kg at most for 1 less belt in the back, a few hundred grammes from cloth seats instead of the leather/alcantara, 1kg at a push for no engine cover, no idea what saving there is from a smaller fuel tank as it's plastic but for arguments sake lets say 5kg, there's still the same sound deadening stuff all over the Cup so there's no weight saving from that, the Cup also has speakers in the front and rear, it's still got airbags etc.
So at best you can account for 30kg of weight saving, what about the other 50kg? There has to be something different with the structure of the car to achieve that, take a normal 172 and a Cup, strip & mod both in the exact same way and the Cup will still weigh less
Another shout out for Chris at EFI Parts, he's mapped mine & quite a lot of other people's cars I know with fantastic results, 1 point worth bearing in mind is that he's very slow to reply to emails because he's so busy and you're basically at the back of the queue until he gets a chance to deal with it but he's most certainly worth the wait