VA Prescriptions

I am a Viet Nam vet with a 40% service connected disability. I use the VA as my primary care provider. I have to pay $8 (copay) for a one month supply of meds. Walmart & Target now fill prescriptions for generic meds for $4, half what I pay. My question is ---- Why can’t the VA doctors write me a prescription that I can have filled at Walmart or any other drug store ? Or, why can’t the VA match Walmart’s price for generic meds ( which all of mine are) ?