Computers are now complicated enough for Computer Science to have turned to some extent into an empirical science - it is impossible for a single person to have in their head everything that goes in a typical computer, operating system, compiler and so forth, so one is often forced to resort to experiment to find things out, it's no longer a theory where you can just reason things out, maybe it never was one in fact, because many simple imperative programs are so complex to reason about.

In empirical sciences, you not only have to have a mathematically valid theory, but you also have to check if the theory fits reality by making predictions and experimentally checking them with the real world. It's the same now in Computer Science, there are so many places where theoretical assumptions might deviate from reality that having a proof is not enough. In fact, you have to have those assumptions to make things mathematically tractable. Imagine mathematicians or computer scientists re-proving real analysis theorems using floating point arithmetic...

In other words, your vision of proofs being enough as long as all the assumptions are part of the theory, seems utopian to me. In fact, even some of the most devoted advocates of correctness proofs have admitted this:

http://www.gwern.net/docs/1996-hoare.pdf