Why Must My Pants Always Shrink???

So, here is my problem...when I am looking for new pants for work, I usually try on multiple pairs in an attempt find ones that are long enough (my legs are relatively long) and not too big everywhere else. I end up buying pants that I think fit almost perfectly in the store. However, when I get home and run them through a wash and dry cycle, they often shrink and are shorter than I desire. Why is it that clothing manufacturers can't just use preshrunk fabric? Since I never know how much they are going to shrink, how can I possibly buy the right size? Therefore, I am stuck with having to line dry most of my work pants so that they maintain the proper length. It is incredibly frustrating, and I have come to the conclusion that it is a way for clothing stores to continue making money off of us because we will continue to return in search of the "perfect" fit. In reality, the "perfect fit" usually only lasts for one wearing.