Eventually it is a numbers game. It is likely cheaper to make large numbers of relatively fast offensive missiles than it is to make an equal number of fast AND

Eventually it is a numbers game. It is likely cheaper to make large numbers of relatively fast offensive missiles than it is to make an equal number of fast AND exceptionally accurate defensive missiles. This is why the US Department of Defense is going in so heavily on microwave and laser technologies. By layering multiple overlapping technologies like this (along with upgraded satellites to pinpoint launches as they happen and help determine the target), you have a much higher likelihood of success. However, if it was just missiles vs missiles, the aggressive side always has a better chance. They just need to have 1-2 successes to cause mayhem. The defender has to have 100% success to prevent mayhem.