Because you no longer have a true phone line in your house is the short answer. Really only DSL which runs on phone lines already will allow you to use a regular phone.
All other things are converting the phone connection from the old analog to something else. Generally most things are variations of VoIP. Because some ISP are implementing solutions that dedicate special bandwidth for voice and have things like batteries to allow you to have phone during power outages you need special routers to connect to those systems.
Mostly your issue is you are looking to buy phone service from your ISP. This tends to be a poor deal they try to bury in the bundling pricing.
Your best bet is to shop one of the many VoIP services...