It's because it's easy and administratively simple, and it's easy to figure out how much you have to earn before you can actually bear the cost. In reality, it leads to a grey area where in the short term you're better off earning less to get the benefit, but it's eminently fair and easy.

And in general, increasing taxes is not easy, and the richer people are, the more able they are to fight against it. So we often create regressive tax regimes despite knowing they aren't very good systems.