Why doesn't everyone pay FICA taxes on all reported income as do low income earners?

Your premise is wrong. FICA is paid on FICA wages (different than reportable income in many ways), and is paid by everyone up to a certain FICA wage limit, which increases with cost of living...currently about 102K a year. If your seldf employed, you pay essentially 2x as much as an employee...because the company of an employee pays half the total 15.3% total tax. (So the owner of your company pays 7.65% FICA for each of his employees).