You forget the other big competitors.
Sun pushed OpenOffice to cut MS's profits from Office
Google and MS are pushing into the Cloud to reduce Amazon's influence
Amazon is creating its own ad network and offering Twitch to reign in Google
Walmart is slowly creating its own global online shopping platform to compete with Amazon
Should Amazon ever have no competitor, monopoly regulations would kick in. But usually, all the other big players will make sure that Amazon has enough competition to not be invincible. It's not fun for small players, but they obviously don't care enough to organize and take their products off Amazon.
Btw, Amazon does not necessarily have less overhead due to Price's law: [1]
>The square root of the number of people in a domain do 50% of the work.
Should Amazon expand into every business, they would be so huge that all their efficiencies and more would be eaten up by the overhead.
[1]https://brainlid.org/general/2017/11/28/price-law.html