it's so people have a "sense of progression" when picking up new items.
Instead of your armor gaining +500 stamina, when you're already on 1.5 million,. you'll just think "meh".
Now instead it's +500 stamina, while you're on 10.000, it'll feel more significant.
That, and there's always internal integer caps to keep in mind.
It's perfectly possible this 15 year old game doesn't have full 64 bit integer support across all parts of the game yet.