Why would dental floss have an expiration date on it?

So you will make another unnecessary purchase.

The wax and polymers that it’s made of can degrade, and become brittle.

It doesn’t expire. The process will still work but it may become brittle and break when using.
The flavor coating may be gone, but that does not stop one from using it.

I’ve been using floss for decades and never had a deterioration/brittle problem nor even looked for an expiry. That won’t change.