That alone is about half of the entire military budget. It's also not like you have a choice to buy foreign military gear, it all has to be designed and made right here, at great expense.
Why? Germany buys foreign arms, Israel buys foreign arms, Finland buys foreign arms and even China buys foreign arms. What's so different about the US?
And then you have a false dilemma - the choice is not between paying 25% of the budget for the army and getting children blown to pieces but between paying 25% of the budget or paying a lesser amount.