Because in a general way you can't say "I want X that will work perfectly until time Y". Instead, Xs are made my a process. That process can cost more or less: more meaning better quality ingredients, higher quality processing, tighter quality controls, whatever. This all yields end results are on a spectrum of quality - a likelihood that the item will last Y time within Z margin of error.
As chain is only as good as its weakest link - many systems will fail with a single broken element. And every time one of those elements breaks, I have a new problem with which to deal. Spend my precious free time figuring out how to do it myself? Try finding someone who will fix it for me, and hope they aren't going to just rip me off?
The example of a home lasting long is especially wild to me. In the US at least, the home is one of the major mechanisms of increasing wealth over lifetime and inter-generational wealth. People frequently buy homes in order to build equity. Having homes that only last a few decades means that they are worth significantly less, and/or require significant repairs and remodels after relatively short time. I know that when I bought my home, which was made circa 1920, I was really happy that, while old, I could be fairly confident it wasn't about to fall over.