The complexity is because the government uses the tax system to motivate behavior. There isn't a mortgage tax deduction just for the heck of it, it is there to

The complexity is because the government uses the tax system to motivate behavior. There isn't a mortgage tax deduction just for the heck of it, it is there to encourage people to own houses. There are long term and short term capital gains because the government is encouraging people to hold investments for a longer time. Any "simple" tax gives up these governmental incentives.