Because this stuff is an important part of our cultural, artistic and scientific heritage. We do not get to decide what the future finds interesting. Therefore, we must preserve as much as we possibly can so that those in the future have the ability to pick and choose. As it stands now, a lot of human history is just pieced together from trash and rubble.

You can go to Rembrandt's house in Amsterdam, visit where he painted, slept, ate, taught, and stored his junk. You can see how he worked, where he worked, what the lighting was like in his house, what the door looked like then and now.

Atari, on the other hand, is just gone. We cannot see their offices (they've been reoccupied a lot), and we have very little in the way of assets generated from the creation process back then. It is currently easier for us to see how Rembrandt worked 500 years ago than it is to see how Atari employees worked 40 years ago. That's not a good state of affairs, considering how influential Atari was on the evolution of home gaming.