People believe this for the same reason they believe a lot of other things with no real intellectual backing whatsoever, it helps justify their worldview/emotions.

A lot of people have a high image of themselves, that all the bad shit that happens to other people would never happen to them. They also see their taxes being taken out of their paycheck every month for taxes and image what they could do if they had that money, but they also fail to realize what they'd lose if the government stopped collecting taxes.

Americans have also been gorging themselves on hyper-individualistic propaganda which has completely diminished the importance of collective action. If something great is accomplished we celebrate the leaders and rarely mention the enormous amount of people that had to come together for it to get done.

So you get a lot of people who have an overdeveloped sense of importance and entitlement looking for a political theory that justifies their actions regardless if it's grounded in reality.

Trickle Down economics and Libertarianism fits this bill. It tells these people everything they want to hear. The individual is all important, taxes are your money not collectively our money, government wastes your money, you are a super responsible person and no one should tell you want to do, you are making a good salary because you are worth more as a person than someone who isn't, personal responsibility means never having to deal with your collective responsibilities (i.e. pay taxes, vote, worry about the poor, not discriminate against your customers with immutable traits, properly take care of your children, correct the effects of past injustices on groups of people who still suffer generational effects this day, etc).