I don't know, I'm not the government. Our army is generally only involved in peace keeping/mediating/shit like Iraq/rebuilding shithole countries after they get obliterated by waves/natural disasters. As far as 'defending' or any preemptive attacking, they don't. So I don't know why.
katie I'm going to explain this fully so that you might understand as I do the necessity of such a conscription as I outlined (one which exists solely for defensive wars).
We have a military because we do not exist as a sole entity on this planet. There are other countries which will not always agree with us and will on some occasions have conflicting self interests with us. When that is the case diplomacy where possible will occur and we will attempt to negotiate. In the event that we can not come to an agreement there will be other measures taken to ensure that either a compromise is reached or one of the self interests will be enacted over the other.
There are several ways this can occur and in modern times this will be a combination of diplomatic manouevering, trade embargos and economic sanctions which will act as a measure of which country has the ability to sway the other in order to ensure their self interests are enacted.
In the event that none of these measures are effective there is a third option. That option is the use of military force. Now I find wars of aggression to be abhorrent and nobody should have to lay their lives on the line for something so ridiculous. However to some the concept of aggressive war is not at all out of the question and is threatened constantly. (namely the countries which we have been aligned with since before WWII the United Kingdom and the United States of America)
As that is the case each country regardless of whether or not a current threat exists must have a military to defend itself in the event that relations between another entity degrade to a point of open warfare.
Now considering that we must have a military (even the swedes and the dutch have far greater numbers of active military than we) we must have one that is worthy of the name. We must be able to defend our country in the event that it would come under attack by a foreign entity who would wish to neglect our self interests and forcibly impose their will on us by use of force destroying our livelihoods in the process.
As such it is our obligation to best be able to defend our livelihoods in the event that it might be threatened in such a way.