Because by it's very nature, tax is theft. Just as people would be scared of being robbed, it is only natural that they should view taxation in the same light. Any one who is not scared of taxes has been psychologically conditioned by governments into submission. This also sets the standard for many other injustices and immoralities to take place in society.

A society needs a form of organization, which is government, and such government would need funding to operate. Any person with self integrity would want to contribute in proportion of his capacity for the operation of this government. But that does not mean it is something that can be forcibly taken.

Taxes are a throwback to ancient/feudal ways of squeezing wealth out of the working people by the monarchs and the rulers.

In future forms of government, there is no place for forced taxation. There are many alternative, morally just ways of raising revenue for a government. These morally just ways however will force governments to be democratic, non-corrupt, just and transparent - so obviously politicians would not want that happening