Because everybody does it, and because the big US corps conveniently have strict regional salary bands. Also, for the top tax brackets (120k+), the total cost

Because everybody does it, and because the big US corps conveniently have strict regional salary bands. Also, for the top tax brackets (120k+), the total cost per employee, divided by net salary, is far higher in the UK than the US. The UK government incentivizes corporations to pay their employees less.