Planned obsolescence is part of it, certainly in some areas such as mobile/cellular phones - manufacturers choose to provide software updates only for a couple of years when they could do so for several more (at least in terms of security/bug fixes even if not providing the full latest versions).
For other areas, domestic white goods (washing machine etc.) I think it's more of a case that most people want things cheap, don't care to do any research, don't consider externalities (machine is really loud and uses more energy, but it's cheap!) so most manufacturers have to cut corners/take risks (reduced QA etc.) or knowingly reduce quality to get the price down. Even though with installation, energy, replacement costs the quality machine is on average the lower cost option over the long run.
Fortunately, as ever, best answer seems to be second hand goods from the manufacturers who still do things properly (or closest to it), like the ~3 year old ThinkPad laptops I buy at about 1/6th the original price.