It is a lack of a better solution.

You absolutely have to control for weight in some way, because size is generally an advantage. Once everyone begins weight cutting everyone basically loses the advantage of weight cutting (fighting at a larger size than your opponent) while retaining the disadvantages (not enough time to fully recover from the dehydration, lack of energy, health issues from severe cuts). But what system can you put in place that restricts weights without allowing cuts?

How do we enforce weight groups?

Fight Day Weighing: You encourage fighters to remain dehydrated shortly before the match to help make weight, so you are risking having starving, dehydrated fighters with no energy engaging in tremendously strenuous fighting that could lead to heart/organ failure, severe complications, and even death. Fighters cut less, but its a balancing act of "how much can I cut without sacrificing too much" and doctors generally abhor day-of weigh ins.

Day before weighing: Same problems, but not as severe, fighters get a day to recuperate back to a normal weight and re-hydrate. Each weight class ends up being "inflated" (155 weight group is made up of mostly 170-180 natural weight fighters) but everyone is still at similar enough weights. The more a fighter cuts the better his strength/size advantage, but he is likely more drained and fatigued from the cutting than his opponent.

Longer than a day to last weigh in: You start to allow fighters to more drastically cut weight to dangerous levels because they have much more recovery time to re-hydrate. So now you have someone willing to get to deaths door 2-3 days before the event, knowing they can spend that time getting back into fighting shape and having maybe 5-10 pounds on their less willing opponent. Eventually everyone does this until people start dying/being hospitalized from excessive cuts.

Boxing, and now ONE have implimented a series of weigh ins in the month leading up to the event. The hope is to discourage excessive cuts, because if you cut for 5 days for a 30 day weigh in, then cut excessively for a 7 day weigh in, then cut even more excessively for a 1 day weigh in, you are basically destroying your body for a month and leave yourself unable to fight.

The problem with that is that you might remove a lot of top fighters in the second highest weight class. If they can no longer make the consecutive weigh ins, they are forced to move up and be the lightest fighters in the heaviest weight class, or permanently try to become slightly smaller.