Because science. You can put metal int he microwave oven, but if it has sharp edges it will spark and it may create big electric bolts. Those might destroy the oven if you let them be in there like that for too long. People have tested various metal things in the microwave oven and it wasn’t do destructive as they thought, but still… they fried the oven.
Best is to avoid metal altogether when putting food int he microwave oven. The oven works be transferring energy from the microwaves to the water molecules in the food. If there are no such foods to transfer the energy to then the waves will return to the magnetron and fry it.
If you have a metal ball in there, then the microwaves will excite the electrons from its surface and heat that thing up like crazy. Don’t wanna be around when that happens.