Because it's nonsense..
Consider a family of 4 with 2 young kids, living in rental with one parent earning NMW then on top of the working partner's £22k(on which they pay £3500 tax and NI), the family also recieve £24k in Universal Credit!!! (That's £628 basic rate + £1.5k housing + £585kids - £600 subtracted for the NMW salary per month).
They can't possibly be paying more tax as a proportion of their earned income than any higher rate tax payer - even if they handed over every penny of the NMW as tax - because they get more than that in benefits.
But even if you take the through-the-looking-glass position of treating the benefits as if they were actually earned income. That family hands over £3.5k in income tax and NI out of £46k they get, then however much VAT etc they pay on what they spend the remaining £42.5k on.
Compare that to a £67k ( top 10% earner) in a similar family with a SAHM. They hand over £17.5k in income tax and NI, leaving £49.4k net salary, then however much VAT etc they pay on what they spend that remaining £49.4k on.
The 2 families have very similar disposable income: £42.5k versus £49.4k. They are going to buy the same stuff, paying the same tax.
How can you possibly suggest that the NMW family is hard done by in terms of tax compared to the top 10% earner, who paid £14k more income tax and got £24k less benefits and ended up just £7k better off?!?
This country's tax is completely nuts. And proof that no matter how much you placate the socialists, they will always, always demand yet more.