Why Is Most Life Symmetrical Externally But Not Internally?
First, I think it worthwhile considering 'Why would internal symmetry be beneficial?' Developmental simplicity jumps to mind immediately. You can also consider relationship to external organs; the stomach and esophagus are lined up with the mouth which is symmetrical about the sagittal plane. Or maybe even balance; the lungs are large organs and if put to one side would likely cause locomotive issues. (Perhaps this is even an interesting topic for another question.)

That said, I feel, at it's core the evolutionary advantage which led to the lack of ubiquitous internal symmetry is space. Simply put, there is only so much room inside an organism and every little counts. Thus, if there isn't a need for a particular organ to be mirrored about a plane then there is a benefit in putting elsewhere: utilization of space.

I think a fantastic example of this is the human digestive tract. The key factor in the shape of the intestines is utilization of space, which directly affects the point at which is connects to the stomach, itself contributing to the asymmetrical shape of the stomach. One could envision other configurations, sure, and nature has. However, this configuration works quite well and the extraordinary use of limited space seems to outweigh all benefits of symmetry.

To directly respond to your questions above:


Question: Why do we not have an even number of each organ so it can be placed symmetrically?
Response: Each organ addresses (or addressed) a need of the organism. Addressing that need with multiple organs working in concert has benefits and consequences, as does addressing the need with a single organ alone. These benefits and consequences are balanced throughout the evolution of an organism.
Question: If we have a single organ why is it not placed in the middle like the brain or bladder is for instance?
Response: I feel space. Again, there are benefits to symmetry but there are many other factors at play. Some of which, it seems, are more important than symmetry at times.
Question: Is there some evolutionary advantage that led to this setup?
Response: I hope this has been addressed - I don't claim to have 'answered' anything, this is a question for discussion.


Other fuel for discussion:

In thinking through this question I found myself able to rationalize why internal symmetry isn't necessary. However, I'd be interested in seeing opinions on why, then, external symmetry is so prevalent.