Honestly Cow, I don't know. I'm perplexed by it these last couple of weeks. Planes sit empty all over the world, airlines are bleeding their cash reserves, and cargo isn't moving due to lack of equipment.
We know these airlines have a few smart people kicking around so I would like to think that if it made financial sense they'd get on it.
It could be that the couriers on this side of the water are so overloaded they aren't likely to be able to move the items for days. Generally when you order something via air you have the expectation of getting it NOW. Maybe the passenger airlines just don't want to deal with what is sure to be a hassle.