As with so many other things the executive branch is doing right now, it doesn’t have exclusive power to do this. Congress sets the rules for how procurement works.
I think you're getting to the distinction between power and authority. Congress may have the authority to decide procurement (I'm not 100% sure on this, going with the discussion), but functionally the president may have the power to force their will through the system.
And nobody listened to the people saying that this concentration of power would be a disaster if the office was ever held by a craven jerkwad. And guess what happened!
