That wasn't always true. The Robinson-Patman Act made it illegal to give preferential treatment to large retailers specifically in order to prevent what we're seeing with walmart and amazon today. The US just stopped enforcing the law (and also anti-trust laws that would have protected local/small businesses) so here we are. At any point the US could decide that enough is enough and fix the situation but we'd probably have to make it actually illegal for corporations to bribe government officials before it stands a chance of happening.