If there were infinite gold bars just sitting on the surface of the moon, it wouldn't be economical to go collect them and bring them back to Earth. No matter how expensive you think any metals are here on Earth, the cost of launching vehicles, rendezvousing with said metals and bringing them back to Earth makes it uneconomical.

An asteroid is much, much further than that but more important than distance is the delta-V required for change its orbit to reach an Earth orbit. So you not only need to get there, which, as discussed, requires in-orbit refuelling with Starship (or any vehicle), but you have to carry all the fuel you need for the orbital burn to bring it back. The rocket equation just kills this immediately.

You really hope you have to get incredibly lucky that an metallic asteroid is on a near-intercept course with Earth that is just shy or going into orbit. The odds for that are, well, astronomical.