Why will most Millenials not become millionaires?
You could ask this question in another way: "why will most people not become millionaires?" It's not that all baby boomers or generation Z is rich.
You are also making an assumption because you can't know how the future will be in let's say 50 years.
Why is not everyone rich?
There are simply not enough resources for everyone to be rich. It's not that people should be hungry, there are enough resources to feed everyone. But simply not enough real estate for everyone to live in a mansion. Some resources are just scarce. Take precious metals for example.