Why do retail companies in the USA, such as Walmart and Amazon, have such ridiculous money and power? They are just retail stores selling stuff from China and don’t even make anything. How can they be in the position and power that they are now in the USA?

If retailing is so profitable why doesn't everyone and my mom start doing it? Why can't I buy directly from manufacturers in China instead of going through Amazon and Walmart?

Majority of US GDP nowadays just a glorify retail store ? It's ridiculous to me companies that just sell stuff hold more power than many manufacturers.