A general question about compiler warnings

People who start programming are usually told to take compiler warnings seriously, or even to compile with the -werror flag which treats warnings as errors. That's because they often are errors. For example, if the types in an assignment don't match, the chances are quite high that you have used the wrong variable name. The result will be compilable but it won't do what you intended.

But when you build packages from source, there are always loads of warnings and you learn to ignore them. Why don't the people who write the code follow their own guidelines?