Why is it difficult to write portable C programs?

Question

I want to know, why is it too hard to make a program run in many OSes, like Windows and Linux, without including glue code. They all share the same architecture (x86), so I

Answer 1

Making your basic C program that computes a little and prints results work is easy.

The problem is that any C program that does anything substantial is going to rely on a lot of services from the operating system, and those differ in many ways. File I/O tends to be similar, but when it comes to graphics, they differ a lot. And that is the crux of the problem.