The problem is that the core of your argument, which is " why are pretty much all new athletes superior to old ones in every single way?" is wrong.
It's not tho, in EVERY SINGLE SPORT where competitors can be compared intergenerationally the newer generation is better. We are talking 95%+ of 'normal' records set being less than 10 years old in sports with hundreds of years of great(!) history.
Like it's not disrespecting gtr as an example to say that he is not as good a player as newcomers today. He was still far better than the competition which was amazing to see, and that's the same in boxing and all kinds of other sport too. Ali would prob get whooped today, because the boxers today are building upon so much more knowledge, i mean if he had the training they had, sure, i guess he might beat them, but like do you see the point i try to make?
TL:DR People don't automatically get better, but the training (and other non-biological factors) does improve, sometimes drastically