It's not really about being stronger

it's about the materials being tougher, less brittle and able to withstand sudden shock loadings from riding over rocks, potholes, ramps etc.
Spokes and alloy rims are softerand tougher than cast wheels and have a greater range of elestic deformation, meaning they can absorb the energy of an impact by deforming and still return to their original shape. So they are good at absorbing repeated large impacts without failing.
Cast wheels are strong, but relatively hard and brittle. They have a smaller range of elastic deformation before they will fail, often suddenly and usually by cracking. So cast wheels aren't as good at absorbing sudden, dynamic impacts.
Don't get me wrong, you don't have to swerve around every pothole if your using cast wheels, both types are fine for nornal use, but spoked wheels are always preferred for any use involving repeated high shock loading such as of road riding.
Right, I'll get me anorak...