Actually steam is not 100% downloaded there are games you can buy on disk that are registered through steam. Dawn of War 2 is a perfect example of this.
And yes it is pirating as you are making money from someone elses property. When you buy the disc you get a license to watch that DVD or play that game you do not however have the right to make money of that or distribute it. Aside from watching it you can make one copy for personal archival purposes that is all. You aren't even allowed to play it in a public place. As for Windows if you buy an OEM copy it is locked to your MAC address and I'm sure if you told Microsoft you sold your disc to the next person they would have something to say about it as well most likely taking legal action against you. As you are making money of their property and their hard work.
Software Piracy is defined as the unauthorized use, reproduction, distribution, or transmission of copyrighted material.
You are not authorised to distribute their software therefore selling it to someone is pirating it.