Different kinds of money; usually it's capitol money that gets delayed for spending later for various reasons, so isn't actually 'unspent'. More like if you were building a house and had payment milestones, you aren't going to pay for the framing job on a date if it's not done yet.
For NP funds, which have that 'cinderella clause' and are only allocated by the fiscal year, usually the unspent funds happens near the end of the year, because something doesn't deliver as expected (or they don't invoice on time). My favourite is when you don't get project funding released until September, so you can't commit to things, and then weirdly companies can't turn something around in less then 6 months as well as if you had 12 months. Usually there is a lot of internal juggling and overplanning so money doesn't get used, but it's a lot of juggling.
Some line items also have contingency for things like extra fuel costs or whatever, so actually should go unspent unless 'shit happens'.
If they want munitions that needs planning and contracts in place, so would need dedicated funding, and not soemthing you can just throw money at with short time frames (within GoC contract terms) and expect things to happen. If we went the opposite way and wandered around with suitcases of cash it might get some results, but would probably get a lot of scams that didn't actually deliver anything (see a lot of UK Covid contracts for examples of what happens when governments ignore all contracting rules.