Many people will throw out their old PCs instead of disposing of them responsibly. Often times these PCs are perfectly usable, or can be used for new sillicon wafers. Some corporations will mass dump old optiplexes, which I take as a bussiness opportunity. With the RAMpocolypse most people can't afford a brand new PC, and with the demise of windows 10, there a ridicolus amount of optiplexes in the bins which perfectly uasable, and even powerful.
Advice on e-waste searching: 1. Try to get e-waste from IT departments of schools or universities, as they spend the most. 2. Look on Ebay as a lot of people will sell lots of computers for low prices. 3. Do not use windows. Windows is higly bloated and isn't as secure as linux. In particular I suggest ZorinOS, and easy to install, easy to use distro.