You're missing out on one very important point: Resources are not infinite.
Sure, robots could conceivably be built that could create anything. But the materials to build those robots have to come from somewhere. You might say that the materials to build those robots come from mines operated by robots, but those mines can't last forever.
Land is a finite resource. Land cannot be free. Gold cannot be free, as it is a finite resource. Etc, etc.
Basically, nothing can be free, as nothing is an infinite resource.
The entire Universe is a resource. Sure, resources are finite. But it will be billions of years before we run up against that limit. Until then, it's just a cost/benefit kind of thing.