because when you reach every kill streak point, they will trigger. So if you get to 500 streaks, ALL previous streaks would have been activated at their correct streak (15/50/100 etc) so its pretty easy to get to 500.
It’s getting to 1000 is a challenge to face big bad Diablo. That’s where the cow level comes handy. If i remember correctly, he gives decent loot.
From what I’ve read he gives infernal machines and a few crafting mats.