The truth is, no matter how many tags were allowed, there are outlier questions that the author thinks needs just one more tag.
The consensus is that if you need more than five tags, you should probably look at simplifying the question. Your question may simply be too big and encompass too many areas of interest.
But, more likely, if you consistently need more than five tags, you are probably adding superfluous tags that aren't really needed. Five is a good limit because it encourages you to pare down the tags to just the essentials. Tagging is supposed to help categorize your question into its major areas of interest. It's not meant to try and sum up your question into every conceivable interest.
If you don't agree, I would suggest that you link to a few sample questions that need more tags. You'll either prove the convention wrong or the users here will come up with some recommendations.