stretches necessity to its limits
Here, necessity means "those things that are needed." In this brief excerpt you provided, I believe the author is trying to question whether or not it is really necessary to answer all the why questions, suggesting that it is impossible. Now, for the "stretching" idiom.
Imagine something like chewing gum. It is very elastic and malleable, meaning that it can be stretched (we are assuming the gum has been chewed a bit). If you stretch the gum far enough, it will eventually snap. In this case, we stretched the gum to its limits (i.e. the point at which it could not be stretched anymore). We apply this idiom in English to any number of things. Here, it is applied to the idea that there are some things we need to do and there are things we don't need to do. It could also be applied to physical objects as well. For instance, if one had a limited amount of water, and a large field, the water would have to be spread very thinly, therefore, stretching it to its physical limits.
This idiom may be confusing, so if you need further clarification, I am more than willing to help.