USA: Why don't airports raise landing fees to make up for low passenger fees?
This depends on where you are in the country and more specifically who controls the airport. A lot of airports in the US are owned by cities (or local governments) in some capacity. As such changing the landing fees often requires a legislative change which is not always easy. For example here is the legislation that covers the landing fees at both major airports Philadelphia Controls



Changing these requires a policy change at the government level and it takes a long time to get just about anything through the Philadelphia government. In many cases these fees are paid to the municipality anyway and the airport never sees any of the money so the fees don't (directly) help the airport. In a lot of cases the fees do (or at least are presumed) to go back to the maintenance and upkeep of the airport.    

Private airports (generally smaller GA ones) can levy fees as they see fit and sometimes do to help with the upkeep of the field. 

What airports or more specifically FBO's and other on field operators do is raise operational costs to use their facilities. Rising FBO fees have been of concern lately and sometimes there is no way to escape them if someone has what amounts to all the parking space on the field. It may only cost $5 to land somewhere but keeping your plane there for a while, getting a pre-heat, maybe even using a terminal is by no means cheap.