Filter only filters out solid particles of a certain dimension. In practice, in a petrol engine, your filter should be good for a few oil changes. It will continue to filter out the crud and won't clog enough to be noticeable. The filter has a huge safety margin in normal conditions.
On the other hand, oil can also be contaminated with fuel and moisture. I don't know of any standard filter which will pull that out of your oil.
Modern oils are very robust and don't 'break down' too readily. If you had a high tech oil filtration system you could probably run a modern synth oil for 50,000kms or even more, however it's currently more economical to just throw out the oil at the predetermined intervals because it's the easiest way to ensure the oil isn't fouled or buggered.
As the costs of oil disposal rise, we should eventually see a point where it's economical to have a very good filtration system, and we will test oil and only change it when it actually needs changing. In fact we may not even change the oil, rather book it in to the local service centre for a clean and re-fortification.