Defense budget is an abstraction. At the end of the day, there are only so many factories with only so many raw inputs producing only so many interceptors per day. You cannot simply increase the defense budget in the event of a war of attrition and then attempt to outspend your enemy. And this is beside the political unpopularity of high defense budgets in peacetime, when they would need to be higher, to build the industrial capacity ahead of when it would be needed.