Some politicians actually care about people. They even care about how difficult it is for people to pay their taxes. So they create an incomprehensible tax code, so that no one knows who pays how much taxes. Complex tax rules allow politicians to hide the true tax burden from taxpayers and also from themselves. Politicians do not feel bad about peoples' problems when they do not understand people's problems. Politicians can relieve the distress which politicians feel about people's problems by making people's problems more complicated.
For example, politicians like to impose taxes on businesses. In reality, businesses never pay taxes, businesses shift their taxes onto their customers and workers. When the government taxes business, profits decline. When profits decline, competition decreases, and businesses raise prices. When profits decline, businesses give their workers smaller pay raises in the future. So the consequence of business taxes is higher prices and lower wages. The taxes are paid by the customers and workers, not by the businesses. Taxing businesses is a complicated way of taxing people which allows politicians to pretend that they are not taxing people.
It would be more honest if the government did not tax businesses, but taxed prices and wages instead.
However business taxes might be simpler than personal taxes, and might result in fewer tax returns for the government to process, and might result in lower adminstrative costs.
Another reason that taxes are complicated is that complicated taxes benefit the people who understand. If you create a tax which you barely understand and which hardly anyone else understands, then other people will fail to use loopholes, and will pay penalties for their mistakes. With other people paying more, rates can be lower, so you pay less.
When politicians consider changing the tax laws, politicians usually consult experts. But experts benefit from more complex tax laws.