Sure, olfactory sensing sensing is "just" sensing of volatile molecules. And we can build plenty of different molecular sensors. Including for things humans can't detect naturally.

But, just like being able to build a load cell and a thermometer doesn't translate to "skin-like sensing", we can't go from "chemical sensors" directly to smell.

For one, things like sensor diversity, density, longevity and manufacturability matter a whole lot. Can you give a robot a weather station type "pressure + temperature + humidity + PM etc" air quality sensor? Yes. Can you give that robot a sensor with enough molecular coverage and fidelity to replace the human sense of smell? Almost certainly not. Can you even get close on a sensible budget? Not with the tech we have.

Getting a system to perform molecular sensing with discrimination against many molecule classes - the kind of operation human smell performs - is very much not trivial.

Now, whether that's worth pursuing at all - I'm still leaning towards a "no". It's a very niche capability.