ELI5 - Why do many new microwaves come with metal racks if you're not supposed to put metal inside a microwave?

I remember when microwave ovens first came out. I was always told not to put metal in the microwave. Of course, I accidentally did a couple times (when I was kid) and the sparks were amazing. How is it that new microwave ovens ship with a metal rack? Are some types of metals reactive to microwaves while others are not?