Simply not true, at least in my case. I haven't been back to my country-of-birth in over 30 years, and have citizenship in the country of my childrens' birth. When the time came to renew my passport, I was informed that my citizenship in my birth country was dependent on whether I had any connection there any more, and since my birth-country has policies regarding dual-citizens needing to justify their citizenship, this was under question. I needed to either go back and live there for a while, or simply give up my citizenship.
Maybe its different in your birth-country and with your citizenship, but its not true that its automatically given and never revoked in every country in the world. Its a case-by-case basis, depending on the policies of the nations involved.