That is what government procurement means. Any competitive process comes before the award of the contract. After that point everyone is generally locked into a one-customer one-seller situation.
Just because it’s how it’s done doesn’t mean it’s not anti-competitive. Government procurement should be sourcing from multiple vendors and requiring interoperability from all of them. Otherwise it’s way too easy for megacorps to lobby their way into a contract and offer products at a loss before cranking up replacement and repair costs by a 10,000% markup since the govt has no choice but to pay it.
That is what government procurement means. Any competitive process comes before the award of the contract. After that point everyone is generally locked into a one-customer one-seller situation.