WordPress is a complex piece of software, just like a car: it looks simple, or at least the problem it's designed to solve is a simple one (moving from point a to point b) but the underlying technology is not simple.
Cars require knowledge to operate and to fix, and so does WordPress. Themes and plugins are like the ignition, the changing gears, suspension in a car: they work well until they don't. Once they don't, you need to know what you're doing to fix them. Good quality WordPress themes, well written, by professionals who know what they're doing are less likely to create issues. Just like buying suspensions from a trusted source will make your car rides safer.