Let's do a basic comparison of 2 parent 2 pre school child households.
Lower income from 2 earners part time - can mean housing association house, subsidised or free childcare, 1 car as can offset hours, child benefit.
Higher income household from 2 earners full time on £40k and £60k - full mortgage and house maintenance (probably astronomical for the same value house) +1.5k, full price childcare to enable them to work full time +2k, probably also need 2 cars and petrol and insurance to both get to work +.4k, might not get child benefit -.2k, likely student loans +.6k.
Conservative estimate for my local area gives an extra cost for the second household of around £4k a month post tax. At 40% rates you have to earn 5.6k per month to make up that difference which is around £67k a year. Extra. To get the same standard of living. This is even London or SE prices.
So assume they spend the same on food, holidays, birthdays, Christmas, one family could be on £30k and one on £100k and, for that period of time they would be in the same financial situation. That's ignoring the extra costs of free time - needing to pay extra for convenience, extra on DIY, not being able to shop around - or the extra NI, or any extra family help.
Obviously these are fairly extreme examples and I appreciate plenty of people on lower wages don't have those advantages. Equally the higher wage pays off in the long run. But if you'd accept the first family on £30k are struggling why wouldn't you accept the second family on £100k are without knowing the full picture?