Even though your ears have their own self-cleaning mechanism, that doesn't mean you don't need to clean them. It just requires more careful treatment so it doesn't cause problems. Likewise with Bitcoin, you can't expect it to be left alone and assume everything will take care of itself. Developers will still implement solutions to improve security or address potential problems, but with a more cautious approach and not just implementing anything. This will ensure everything continues to work as intended without causing problems to the network.