That would discourage people from getting married, which would also be madness.
I was on the receiving end of the injustice you cite though.
Prior to my retirement, I was fortunate to earn over £125k per year most years (and my wife did not work), which meant that we got no personal allowance whatsoever, and paid circa 85% of my pay at 40% tax rate. Vs my neighbours who worked for the NHS, earning about 60k each. That meant they both got a £12.5k personal allowance, and paid nearly all of their income tax at 20%.
Although our households earned the same, the difference in our net income was stark. £93k take home for them vs my £78k. I know I should not complain about such a figure, but our houses are the same - we live on a housing estate - and there is 2 of them and 2 of us. Yet we were vastly worse off.
To put it another way, on these figures, they were around £60 per day better off than us, just because of the tax system. Galling, to say the least. And they got a much better pension and more holidays.