They only work up to the point when they don't. Amazon only plans part of the economy of its workers. It doesn't have to plan for what they eat, where they live and how they spend their money in general.
In a capitalist system companies, trying to maximize profit, will try to expand until it is efficient to do so, but not more or smaller competitors will take over.
The USSR was basically constantly in a Great Depression. According to the US definition of poverty almost all its population was in extreme poverty.
The Great Depression was a Government failure, not a market failure. They didn't print enough money basically.
Planned economies do work, both historically and today. They are immune to the business cycle (as the USSR didn’t experience the Great Depression) and can be more efficient even within capitalist economies (witness Amazon, Apple and Walmart). I recommend “The People’s Republic of Walmart” on this topic.