Two theories:
- When freewheels ruled the roost most freewheels were of a decent quality.
Now when F/Ws are mostly found on cheaper bikes, I suspect the average quality has gone down.
- Pretty much all F/Ws I’ve seen have cruder seals than cassettes, perhaps making them more prone to contamination.

Don’t think temperature expansion is the main culprit. One steel vs another steel is generally very little difference.
Also, a F/W bearing only spin when coasting, when it only sees the residual chain tension. It has a fairly easy job. Assembly has to be way off is the tiny changes of thermal expansion would the the thing that makes it fail.