Why does Seltzer water have an expiration date?

B/c carbonation fizzles out.

I imagine it would simply go flat. They are warning you to have it by x date or you'll be drinking still water.

Seltzer gets stronger and angrier the longer it's kept bottled up. If you drink an expired bottle, your guts will explode. Mr. Seltzer designed it that way on purpose, so you'd always need to buy new seltzer.

Ok I’m willing to drink flat seltzer.

I bought a few cases a few months ago and see some of them expired. I’m wondering should I throw them out or not.

It goes flat. Same with any carbonated drink near or past expiration date.

Yes. Dairy deaths are at an all time high!

In plastic bottles?

Then the expiring date is not about the water, it is about the bottle. After a while any plastic bottle will start evaporating unhealthy ingredients.

R7 cans

A friend did this and we had a celebration of life instead of a funeral.