That's a pretty big assumption. How do you know, for example, that Google doesn't reward pages that have relevant outbound links? If you were Google, mightn't you view relevant outbound links (especially relevant outbound links that were used in context, not just on a reciprocal-links page) as a "signal of quality" or, at the very least, as a signal of relevance?
PageRank is just one piece of a very large puzzle. Most of the other pieces are kept under wraps.