Like bro it's genuinely bullshit. Could've just said that it was discovered to be more efficient to use magic instead because it costs nothing.
Magic costing nothing would make way more logical sense. They could easily make it that unending hordes of monsters just overwhelm military logistics. It's completely reasonable for something like this to happen. Modern militaries are NOT equipped to fight endless wars of attrition like it's Warhammer 40K. They are prohibitively expensive.
All it would take is monster outbreaks weekly of 10,000+ combatants to quickly overwhelm most world militaries and police forces. Add in magic and common fantasy status ailments like poison and you'd be seeing unsustainable casualties among military personnel within months. Various monsters inhabiting cities and sewers would mean months of block-to-block fighting for infantry. Add in even monthly outbreaks of additonal monster attacks and humanity would be pushed the brink.