Ever go to a go kart track and notice that some karts are faster than others? They're all identical as far as the eye can see, but in reality tons of subtle differences pop up.
You can have spec bikes, but there no way they'll all be tuned identically, all have the exact same lubrication in all the bearings, all have the exact chain tension, all have the axles torqued identically. All the derailers built exactly the same... One bike will get inevitably have an advantage over the others.
Why? The whole point is to standardise, I thought? They are already _not_ accommodating preferences for eg recumbent bikes. So what measure are a few more preferences not accommodated?
In any case, you can make a bunch of different official sizes.
No, the main point (as already explained by someone above) is that this competition is about the cyclists, not equipment. The idea is quite simple, but leads to complex, sometimes somewhat arbitrary rules, but they in the end work quite well to regulate the competition.
No offense, but you're clearly someone who doesn't know much about cycling, but are insisting that the cyclists (competition organizers) are "doing it wrong". Arguing with that is tiring, so I won't continue here.
I genuinely see the point you're trying to make, but fitting a bicycle is like fitting an article of clothing. It's is laughable to suggest clothing be one-size-fits-all the same way it is for bicycle geometries. It doesn't compare.
In that case, you should probably give everyone the same 'official' bike.