Tubes aren't designed to be stretched; they are simply there to hold the air while the rim and tire contain the pressure.
Consider what happens when you blow up a long thin balloon.
* First, the loose balloon inflates until it is uniformly full but with little pressure.
* Then, one part of the balloon (typically either one end or the other) will start to bulge.
* Then that bulge will get larger.
* Then the bulge will spread along the length of the balloon.
* Once the entire balloon is uniformly filled, it starts to uniformly inflate.
* Then it pops.
Exactly the same thing is happening with the tube.
* You inflate it until all looseness goes away.
* Then the pressure causes the weakest part of it to start to bulge.
* etc.
But tubes aren't balloons, and they really aren't meant to be inflated to the point of bulging.