Just like anything else made of rubber (or synthetic equivalent)...
Turns to dust after enough time passes.
If one valve stem goes you might as well replace all of them, assuming all
are the same vintage.

I had this happen on a project car- one day right front tire was flat,
inspection showed a bad valve stem.
I walked over to the other side of the car and wiggled the left front valve
stem- came off in my hand and tire fully deflated in about 15 seconds.

I just laughed and headed off to the parts store.