One answer is that people do not realize the irrelevance of a test ban to arms control. Yet another answer is that there are people who view a test ban as a means of promoting nuclear disarmament by making U.S. weapons less usable. They know that without testing, the U.S. arsenal will become less reliable and safety will not be improved. The U.S. nuclear arsenal will become less reliable because we will not necessarily discover problems with the aging weapons, nor will we be able to be sure that the fixes we make actually work. Historically, nuclear testing has revealed such flaws a number of times, necessitating redesign or alteration of affected weapons.
Nuclear testing is also imperative in making safety upgrade.
Technological progress has enabled a number of features which, if incorporated into nuclear weapons, make them safer and more secure. The same logic for testing applies to nuclear weapons. For example, since some weapons designs were introduced into the stockpile, insensitive high explosives were invented. They will not detonate easily if involved in an accident. We do not know what safety technologies may be discovered in the future, and we should not foreclose now the option of adding such measures to our arsenal.