Scope and scale is important here, the amount of junk mail from business interests outside of my immediate region is not very high. If physical mail were free and you could send it from anywhere in the world, junk mail would be so much worse than it is. You couldn't run a lot of internet scams at the costs of physical mail and be profitable.

Probably not because even if the postage is free the paper, printing, envelopes, etc. are not.

How many pieces of physical junk mail do you get per day? Now how many spam emails do you get per day? Include the stuff that lands in your spam folder, because we're talking about cost to send junk mail here.

I'm willing to bet the latter is much, much higher. It certainly is for me.