Because spending money needs to be authorized by Congress through law. A president can't just issue an executive order and say "You don't need to pay this government guaranteed debt -- the taxpayers will pick up the tab. Because I said so."
There have been several executive orders that were found unconstitutional:
- Executive order that landlords can't evict tenants for non-payment of rent during covid
- Several executive orders about vaccine mandates
- Executive order imposing a moratorium on oil and gas leasing
etc.
