You seem to think that we should request code, because without code, any crazy result, be it fraud or honest mistake, can be slipped into the journal. But this is not so. Including code is a nice-to-have feature, not a must-have feature. The other answers silently assume this and explain the (good and not-so-good) reasons which lead to the current situation of uncommonly included code. I think I can complement them by explaining why it is not a must-have feature. 

For theoretical results, you don't need any empirical tools like code to reproduce them, as others mentioned (e.g. proving that an algorithm has a better big O behavior than another). Of course, there are also empirical results, which cannot be replicated that way. 

But your reviewers will have an expectation of what your idea will result in. If the current best performance for wugging zums is 3 zums/s, and you add a minor tweak and report 300 zums/s, your reviewers are supposed to notice that your claim is unusual, and do something (possibly demand to resubmit with the code). This is not foolproof, but with multiple reviewers per paper, it is effective, because the magnitude of most empirical results is predictable once the reviewer sees the idea and understands how it works. 

For this class of paper, both honest and dishonest mistakes have a good chance of being caught, with bad results for honest scientists (reputation loss, especially if caught after publication) and worse results for dishonest scientists (end of career if proven!). Moreover, the graver the mistake (as measured in the size of error), the higher the chance of being caught. It is less likely that you will get caught if your algorithm manages 4 zums/s and you report 5 zums/second, than if you report 300 zums/s. So, scientists are disincentivized from submitting incorrect papers, leaving less incorrect ones in the submitted pool, and the reviewers catch lots of the remainder. 

There are cases where it is totally unknown why an observation is the way it is, and in these cases, it is very important to describe the exact test setup perfectly. But I have never seen this kind of paper in computer science, it is associated with natural sciences. So no code there. Even if you got such results in computer science (e.g. you observed that users are capable of reading a 12000 word EULA in less than 30 seconds, which contradicts common reading speed observations, and you have no explanation for it), it is unlikely that including the code you used to obtain the result will be pertinent to replication. 

To put it together, among a large mass of computer science papers, the theoretical ones and the natural-phenomenon-observation ones don't need code inclusion for replication, and the remaining ones will contain only a low percentage of incorrect-but-uncaught papers. Aggregated, this leads to an acceptably low level of incorrect papers being submitted. Requesting the code to go with them will increase quality for one class of paper, but it will be an increase of an already high quality level. It is not that not having this feature makes the current quality too low. 

Why are papers without code but with results accepted?
The old bike is a Cannondale R-600 purchased in 1999. 


Original key components. Frame, fork. (Aluminum frame, carbon fork)
Ridden for several thousand miles. 
Well maintained. (Various parts have been replaced and upgraded over time.)
A few visible minor dings here and there.


Question: Is the frame/fork safe? I can sell it or trash it. 
