>why not?
lets put it this way.
we are a capitalist society.
A Capitalist consumerist society.
our economy revolves around people buying goods and paying taxes.
the standard of living that this country has enjoyed since WW2, was a direct result of a booming industrial sector and an abundance of industrial/factory/labor jobs.
those kids of jobs pay well, and allow an average person to have some money left over after living expenses to maybe buy a new car, or buy a new BBQ grill, or hell, that lawn mower is getting kind of old, let's buy a new one.
This man works in a factory that makes Ford cars. it is a good paying job. the executives at the top obviously make far more, but the worker still makes a good living. with the money he earns from his job, he can raise a family, and then have some money to pay for goods for his family. he can buy his son some toys and his daughter some dresses. maybe at some point, he decides he likes that new sedan his factory built, and so he decides to buy it.
that is the kind of living standard that the civilized, industrialized world knows.
the majority of the people in the "middle" will have some kind of job like that, while those at the top, have a higher education that allows them to make more money in an exclusive position, and the lower end person maybe works at a retail store for a living, or owns a small family owned shop.
TL;DR, service jobs don't pay enough to live off of properly, unless you artificially raise their wages like they did with McDonald's. they are not jobs that were ever meant to be lived off of entirely.
And on the other end, you have higher end intellectual and educated jobs like CEO, scientist, lawyer, etc, but not everyone can have that level job. they are traditionally exclusive jobs for a reason.
You need that middle ground, otherwise you go back to pre-industrialized society, where anyone who isn't rich, is just poor.