This is a really awesome description too. The only thing I know about compilers is that I implemented one for class (without fancy optimisations) and I am surprised any software works, ever. Compiler are just ... mindbogglingly complex things. Almost as much voodoo dark magic as engineering.
Oh, and many serious compilers can compile themselves, so bootstrapping is a pretty good test too.
Another reason is that compilers simply must* work for software development to continue, so people have spend the necessary amount of energy to get implementation and tests "right".
(*) There are always exceptions, like when a compiler auto-parallelizes code and introduces a race condition that is rarely triggered. But those cases are blessedly rare.
You have on your hands a compiler that you can no longer think of a program for, that would produce an incorrect result. You're happy.
Then your friend comes along and the first thing they try behaves incorrectly.
The problem isn't even that the compiler would crash and burn, it lovingly compiles the input program into something that does the wrong thing!
[1] http://gcc.gnu.org/wiki/summit2010?action=AttachFile&do=... [2] http://embed.cs.utah.edu/csmith/