How would your client know that someone followed you, unless they intentionally post their kind 3 event to a relay you've told your client to watch?
Primal can give you this information because they run their own server that aggregates this information from known relays. Most mobile clients don't have this type of back-end infrastructure, and run everything locally on your device. Trying to aggregate that kind of information locally on a mobile device's data connection would be ridiculous.