I don't think it's a matter of can't (though with 2V [haha! I said I was going to use that], I'm not sure I'd trust them to be able to pull it off in a smooth manner...) but rather just that the benefit isn't worth the cost.WHY can't Amtrak assign specific seats? The airlines do (except maybe Southwest, but that's their choice).
I agree with the_trav (someone called him that elsewhere, and I think I'm going to appropriate its use) that I would rather not have preassigned seating on long-distance trains. The equipment is simply too variable. I want the freedom to move to another row if the window is aligned better or it has an available power outlet or another coach if the HVAC is malfunctioning or there's a noisy third-grade school group.
Preassigned seating would work fine on Acela, where the cars are pretty much mostly uniform, but on any other train, you can get stuck with all manner of different equipment.