Because the only way you could weaponize a bacteriophage is by deciding you want to kill one specific bacteria strain in a person.

You could make a bacteriophage to target my favourite brewers yeast, but selecting a single strain of ecoli in my gut wouldn't harm me any more than eating a giant bowl of extra-extra-spicy salsa.

They don't have the same mechanisms as other viruses. They physically cannot harm eukaryotes.