PAN-DB does have the ability to categorize IP addresses, though the Palo Alto Networks device purposely do so in a limited fashion.
There isn't always a 1:1 mapping between an IP address and an actual host. Given the possibility of differing content, the Palo Alto Networks device does not categorize, as it may affect policy. For those that are a 1:1 mapping, if there is enough content for the Palo Alto Networks device to determine what the category is, it will. However, note that IP addresses can often change, which is why these do need to get refreshed from time to time. Additionally, some sites often deliver dynamic content, which also makes it more difficult for the Palo Alto Networks device to determine a category, which is sometimes why they remain unknown.