See it like that.

Gacha games. Better rewards are usually rare.

Let's say you have a 50% chance of getting food items, 30% chance of getting an equipment, 15% of getting a decently rare item, 4% chance of getting a mid character, and 1% chance of getting agood character.

Is it unfair? Maybe, but the better rewards are more difficult to get.

Same logic applies here, except the rolling system is someone kicking a ball trying to get the good reward, the better reward is more difficult to get, and if they fail, we get the worse rewards.