Reliability is hard because we continue to move the goalpost. I can write a pretty reliable program to compute sin/cos tables or to sort a phonebook or to do some basic projectile trajectory. But those aren't even table stakes anymore. Today a first year undergrad student might be expected to write a program that in 1970 the most advanced programmer in the world couldn't do with a large team and millions in funding.
I hate the analogy to other fields of engineering (and this author doesn't fall down that trap, but plenty do). Bridges haven't changed in the past 10 years the way programs have (in terms of functionality or complexity).
And lastly, this is IMO, a corollary for why ageism is so persistent in our field. Experience means very little. Why? Because I can capture 70% of your experience in your code -- in your library -- and build directly on top of it. I don't actually need you. There are few other fields where as much of your experience can be captured in something so reusable (despite our constant protests that code isn't reusable -- it's more reusable than any output from my plumber).
Programming is terrible because we obsolete ourselves by moving technology out of our own grasp. The fact that newer programmers don't understand the foundation they build on is irrelevant, since few need to look behind the current. Unfortunately, I think it is simply the nature of the beast. It's why we love it and do it -- and probably why we'll all eventually stop (or at least move into management).