S3TC analysis, page-7

"I doubt anyone would consider a couple both on $85k as rich, but the $170k single earner is considered rich by the left wing media despite having much less to support his family."

There are not enough votes in play to address this glaring inequality at a family level.
It essentially arises from the dual income family having 2 bites of the tax free threshold.

Historically there were some rules that minimised this inequity
You could claim a tax deduction for a non working spouse.
FTBB had a much higher cut off level for single income families.

These rules have been eroded as 'budget savings' by both liberal and labor goverments.

The taxation of income is clearly a 'vote management' exercise rather than a exercise in taxation equity.