That adding a motor would be upgrade and tinkering of a bike. So why should it not be allowed? Isn't it entirely in same spirit?

Do you want me to enumerate the ways in which a bike is not a motorbike and the parameters in which people who are interested in bikes operate in their hobby or profession? Or are you asking why professional racing puts guidelines around what types of bikes can participate?

You’re being disingenuous and down right annoying. By your logic why not just use a F-18 and win the race in 10 minutes? Stop being asinine.

Not sure if you honestly believe this or just trying to be "edgy" but because a motorcycle is not a bicycle.

I assumed it was a reference to this - https://en.wikipedia.org/wiki/Mechanical_doping

From a bit of reading, it sounds like there's a fairly widespread impression (or at least suspicion of) cheating. And that enforcement is quite weak, because the authorities care more about preventing public scandal than about preventing actual cheating.