Starting with the IBM PC in 1981, it was made with off-the-shelf parts and you could get not only the schematics but the BIOS listing as well. It was very modu

Starting with the IBM PC in 1981, it was made with off-the-shelf parts and you could get not only the schematics but the BIOS listing as well. It was very modular so one could be upgraded piecemeal. But at the same time, Moore's Law meant that CPUs would get smaller, faster, and more capable about every two years, which generally meant a new motherboard. This got the buying public into the habit of basically buying new computers every few years anyway. By the time CPU speeds topped out around 2005 or so, the companies selling computers were just expecting people to buy new computers, people were (and still are) price driven [1], so there goes modularity (cheaper to integrate everything rather than made it out of modules---less to break as well). So by the time we got the point where modular computers actually made sense, there's no incentive from either side to actually do that. [1] Want to pay more for more comfort on an airplane? Forget it. There aren't enough people who want that and travel for airlines to even think of targeting that demographic.