Could the terminal length of components like resistors be reduced?
Through-hole components, other than power components, are not really a hotbed of innovation since they've largely been supplanted by SMT parts as of 20-30 years ago or so. The equipment, such as cutting machines, end cap crimpers and welding machines, is being run down and not replaced most likely. 

In some old designs, resistors were mounted vertically, which required the long lead on one side anyway. In fact you could, at one time, buy resistors preformed to that shape with the long lead covered partially with lacquer. 

Further, the standard packaging to fit automated assembly machinery is tape and reel, or ammo pack (Z-folded taped parts shipped in a cardboard box), which requires long leads with tape at the ends. Again, that stuff (referring to through-hole automated stuffing equipment) is generally not being replaced (image from here. 



Bulk packed resistors might be used for manual assembly in very low end products, assembled in places with dirt-cheap labor rates, but that's hardly a growth market. 

The leads are very seldom made of copper, rather they're made from plated mild steel, and you can easily confirm that claim with a magnet. Some SMT parts use copper leadframes rather than steel to increase thermal and electrical conductivity. 

Back in the old days, for manual assembly, one could order resistors cut and formed to fit a PCB for a nominal additional cost (not through distributors but going directly to the manufacturer). It would cut down on shipping cost. The manufacturers ran their standard parts through a forming and cutting machine, but I think they bypassed the tape part (my memories of through-hole resistor factories is getting a bit hazy after all these years).

It might be interesting to quantitatively compare the waste percentage (by weight or volume) of modern parts like 0201 or 0603 resistors vs. tape and reel resistors. I'm guessing it might actually have gone up.