Your CFI LICENSE has an expiration date. Just like a driver license, state medical license, hunting license, etc..
Your Airman/s Certificate is a Certificate showing that you have proven your ability to fly to a minimum standard of competence. It never expires. Nor does your birth certificate, BS/BA, MD/DO, etc.
So what is the difference?
Well, the lines are blurring, but in general a license is a contract between you and the government which gives you the RIGHT to do something - i.e. drive, teach, practice medicine, hunt/fish, dig septic tanks, sell beer, and so on.
Because a license gives the contractual Right to do something that others are forbidden, for the state to cancel that Right they have to show cause, perform due process, etc. The courts may be used to mediate disputes between the license holder and the issuer.
A Certificate is a Privilege granted at the discretion of some power - and that is the huge difference.
In our case that is the FAA Administrator. His issuance of an Airman's Certificate does not grant you a Contractual Right. You exercise your privilege to fly at the pleasure of the Administrator. He may cancel your Certificate at his discretion.
Early on the Courts refused to interfere with the FAA Administrator in the discharge of his duties. In more recent times this is changing and the Courts are becoming more assertive.
In an aside: I have a Certificate that states I have met the qualifications to be a Physician. It has no expiration date. I will always be a Physician.
However, to actually practice medicine I have to have a state license which does have an expiration date.
