My running joke for the last couple years is "Nobody knows how to write software anymore" (even about a bug in my code). The problem is almost certainly complexity. Apps are like a house of cards, and we keep adding to the stack with new features, external dependencies, etc. My apps 30 years ago used one language, one library, one process, one thread and shipped on one platform. Now it uses Java, JavaScript, CSS, HTML, SQL and has a dozen linked libraries, talks to a couple servers, has dozens of threads, and runs on desktop, browser, tablet, phone and car console. It's a wonder anything works at all.