The inside is made of metal to keep the microwaves in: microwaves reflect of metal, so they bounce around inside until the food absorbs them. The reason for not putting metal inside it depends on the metal and its shape, somewhat. Big chunks of metal reflect microwaves; small chunks tend to absorb quite a lot. Early microwaves had a problem that metal tended to reflect the microwaves back into the magnetron (the think that makes the microwaves) and overload it, causing it to burn out. That is fixed nowadays. But if you put a plate with gold leaf on it, that will absorb a lot of microwaves and burn out. The other thing is that the metal will shield the food from the microwaves. If you put food in an aluminium tray into a microwave, only the top will get cooked; the tray will prevent the microwaves from reaching the bottom.