How old is modern boxing? About 150 years old? If it ain't broke, don't fix it.

There was a time when fighters adhered to the established weight divisions.
Titles were won in established weight divisions.

Weight gain has always been an issue from the previous day weigh-in to fight time. But it wasn't until the rise in the use of diuretics to achieve artificial weight loss, that fighters like the scumbag cheat Cesar Chavez Jr. made a habit of coming into the ring 20-25 pounds over the weigh-in weight.