I'm not sure why. I think it's partly because I'm afraid of them bursting but that's not the whole reason. I just don't like them is all, containers might not be any better than them but I'm more comfortable with them.
Well, I wouldn't worry about them bursting. I crashed on my mountain bike and landed on my back, and all 220lbs of me at the time, didn't even cause a leak in my camelback at the time, much less burst it.
As far as weight is concerned, I guarantee you a hydration bladder doesn't weigh more than the same size canteen or bottle. So, you have a 2 liter bladder, doesn't mean you have to fill it to the top. I routinely fill mine with only a liter of water for short hikes or biking trips. So, your weight argument is invalid.
Lastly, if you don't like them, then just take it out of the pack, the pack will function just as well without the bladder, I promise.
As for your question to why all the packs are made to hold a water bladder, well, because the consumer wants it, so companies are going to make what sells.
So, if you don't like them, that's fine, it's a personal choice, that's why they didn't stop making canteens and bottles.
So, you may or may not like them, but I don't think you could show me one good reason why a bottle or canteen is better than a bladder, besides personal preference.
P.S. Why do I like bladders? Because they are a hell of a lot easier to use, just suck on the straw, no need to unscrew or flip up caps. They also make it easier to carry the water's weight, and make the water more pack friendly. Let's face it, water carried right against your back is a lot more comfortable than a canteen or bottle making you lopsided. And when you put your bottle or canteen in a pack, they are more cumbersome and harder to pack around.