If you mean building a computer from fundamental components, you can still do that too. It just won't feel quite as magical today as it did in the 70s or 80s because of the ubiquity of modern computers.
I guess I meant building a computer with a chance to make real decisions. E.g. busses, interrupt architecture, memory controller etc. Now its all integrated. You use a certain chip; you need a certain chipset to support it. Your project becomes cut-and-paste of the single supported design. Why do it at all if you can't change anything significant? "I built it myself, but no, its not different in any important way from one you could buy off the shelf cheaper"?
