What field are you talking about? A CS paper describing the design and performance of a computer vision algorithm is different from a sociology paper that used a spreadsheet to crunch demographic data.


  Do most journals / conferences just "trust" that people who submit the paper really implemented the theory and got those exact results?


Yes. The presumption is always that there is no scientific fraud involved.


  I always had this idea that any experiment should be reproducable by others else it's not scientific justified.


If the algorithms are fully described in the paper, then the result is reproducible. To reproduce it, you have to reimplement the algorithm.


  I just started reading some papers and thought now let's look at the code and was quite astonished that most of the papers don't have any code to look at, while claiming some performance or being better than other papers.


Presumably the better performance is because the algorithm described in the paper is a more efficient algorithm. For example, when sorting a large amount of  data, a quicksort is a better sorting algorithm than a bubble sort. The quicksort has O(n log n) performance on the average, while the bubble sort has O(n^2), and this is true regardless of the details of the implementation.

Why are papers without code but with results accepted?
What is this variety of kidney beans called in English? We call them "Chitra Rajmah" in Hindi.
http://www.helloorganic.com/Rajma_chitra.html

