Different individuals can earn different incomes because they?

Question

live in different areas with different employment opportunities.

are discriminated against.

have different levels of education and experience.

Any or all of the above could be reasons.

Answer 1

Live in different areas with different employment opportunities, and have different levels of education and experience.

Answer 2

And have different negotiating skills.

You don't get paid what you are worth.  You get what you negotiate.