First, I would point out that surnames are words coming from X language which is why surnames are never ( at least for most European and American cultures, I can't speak for some other parts of the world ) a good way to find one's origins.

As you said, many surnames are from locations or trades, from nicknames as well ( such as Small, Bold,...Etc ) but more importantly they are from languages. That means that you can have the same surname ( the same meaning ! ) in one single country but written in many different ways and sometimes with spellings that have nothing in common with the others.

For example in France, there are some family names that have more than 40 different spellings because they come from different languages and dialects which were once spoken in France ( such as Breton, Alsacian, Catalan and so on ) and now look like completely different words though they could mean the same thing. That's one reason. Though my example was France, many other ( European ) countries fit in this example.

Just to give you a number, in 2003 in France a study made by Marie-Odile Mergnac in her Encyclopedia of Surnames showed that around 300,000 French people are the last ( or close to be ) bearers of their surnames. That says that these people have last names which are now ( or always were ) rare and most likely words you will not hear often ( so no Rivers, Woods, Forests,...Etc ). There are as many possibilities for surnames that there are words in all the different languages. That gives a lot of different surnames.