You might want to add functionality such as caching or business rules.
A better question would be why not write a module or class? There a pros and cons to either, but advantages include: better monitoring and alerting, easier deployments and rollbacks, callers can timeout and use fallback logic, you can add load balancing and scale each service separately, it's easy to figure out which service uses what resources, it makes it easy to write some parts of application in other programming languages, different teams can work on different parts of the application independently as long as they agree on an API.
Strictly this is not necessarily relevant. You can easily roll that table hit into another db hit you were already making. Can't do that with services.
Yes, but instead of making it a whole new service, you are probably already using a database and can use that service for this functionality as well.
But since asking the question I've realized that if your application already needs a huge amount of servers because it simply gets that much traffic, then putting something like this in its own docker instance is probably the simplest way (it might even use postgres inside it), if those boundaries change now and then.
A better question would be why not write a module or class? There a pros and cons to either, but advantages include: better monitoring and alerting, easier deployments and rollbacks, callers can timeout and use fallback logic, you can add load balancing and scale each service separately, it's easy to figure out which service uses what resources, it makes it easy to write some parts of application in other programming languages, different teams can work on different parts of the application independently as long as they agree on an API.