A lot of the second-hand computers go to charities, and end up in schools of developing countries, etc. These are often places that wouldn't be able to afford computers otherwise.
On long term, the people who learn computing this way may be a market for new computers, and that makes it good for computer manufacturers.
Besides that, computers in a landfill is a terrible idea, ecology-wise.
Do you have any data to back this up? I've been tasked with getting rid of old computers before (at a large company) and it was a very painful process. Schools wouldn't take them. I can't even imagine trying to send them to another country! We ended up actually paying someone to haul many of them away.
On long term, the people who learn computing this way may be a market for new computers, and that makes it good for computer manufacturers.
Besides that, computers in a landfill is a terrible idea, ecology-wise.