For many people it is. They cannot see another doctor if they want to continue doing things like eating food. If they're lucky enough to be insured at all.

Could you explain the mechanism of this? I am unsure how this situation could arise. There are seemingly infinite GPs in-network on any insurances.

Is their GP the only practitioner in-network for them? Their network has one member?

No, but changing your GP normally requires a new patient consult.

Many GPs aren't taking new patients (or at least from whatever insurance you have) or the initial new patient visit may take 4+ months to schedule.

GP is not a particularly popular choice for doctors as it pays significant less than any specialty does.

Depends if there are other in-network GPs in a range where OP can reasonably travel to.

And who are also accepting new patients.