But why not build an even bigger space station by combining payloads from multiple launches?

Possibly, with bigger modules, you could build it in a way that each module is fully redundant, has its own power, thrusters, docking, etc. That way you'd have a fully operational station from the first module, and great redundancy if there was ever an accident.

We hardly have any use for the one we have. The best that could be done is one cheaper and better suited to the few uses we have discovered.