The correct answer is because the UK is still an extremely class based society, and the upper class is making sure that it stays this way.
Forbes also reports the British monarchy "contributes nearly £1.8 billion to the UK economy" annually, including £550 million in tourism.
So why are tax payers putting money in their pocket?
It seems they're merely a tourist attraction, like Disney Land. But We don't give them millions of dollars a year.