Sometimes the answers also come for political or project management reasons, as much as physics and scientific reasons.

The ISS is slightly risky. It is hard to know how much longer the ISS will be there for. It is currently funded and authorized through 2030, with deorbit scheduled for 2031.

There are other reasons that could potentially shorten the lifespan of the ISS further.

A sample return mission might take 10-15 years (or more) to plan, get approval and funding for, to build the hardware for the mission, to launch that hardware, and the time to get to and from the sample location. (In addition, often these are tacked on to other missions; which might include a science package that has more duration.)

If a mission would use the ISS as a critical component for it's return journey, that means that the mission assumes extra risk based on the likelihood of the space station being available for it's role in the mission at the end of the mission.

And while we can certainly assume that the Space Station will be funded for longer... the PM and approval teams cannot assume this. This means that if the project is expected to take 10 years, the project cannot use the ISS, because on paper the ISS will be gone by then.

So the question becomes: does using the ISS as a return link outweigh the risk and scheduling complications? Given u/electric_ionland's very nice response, my guess is "no".

Note that I don't work for NASA, any governmental agency, or any governmental contractors.