I had a user ask me this question. We know that cars break down, but that's because of something physical (unless software is involved!).
I tried to answer that software is a much younger industry, but the user countered with "didn't the automobile industry become much more stable than and reliable with less people?".
I also tried to answer that software is more complex, but the user countered that there are many thousands of parts that make up a car. People that design and build cars generally just know their component(s) very well, but they still all end up working together as an end result.
So, why isn't software as reliable as a car?