Also because they need equipment not cash. If the US would give them money, Ukraine would still have to get equipment somewhere.
Now the US is clearing out old inventory that is at the end of its shelf life (explosives have a shelf life) and the Ukranians use them to blow up Russian military instead of the US paying a company to dismantle the munitions.
The US charges the Ukranians current book value (low) and reimburses the military from the aid budget. The US military then uses this extra cash to buy new toys.