I don't know if your service will be around later on. I don't know if you will change the licensing terms. I don't know if your server might be off for maintenance.
Those are basic things, but what it comes down to is that I don't have control over your offering. Why would I add so many dependencies, when I can do it locally, and KNOW it'll work.
On top of that, there's practical questions-
Why add latency for a round-trip request, when I could do it locally quicker? What if I need to tweak your formatting? If it's a local lib, I can open the source code and add new types myself.
Sure, I might need to change code if a new type of telephone number is invented (??) but people release new versions of libraries, too.
It's cool to learn how to write an API, but I can't think of many sane people who would use yours in production.