As the decades went by, people wanted the government to do more and more things and it needed more and more money to do it. Look how many more things the federal government does today versus when income tax was first introduced.

"Inflation brought everyone into the taxable income brackets" has very little to do with it. A significant portion of the population still doesn't make enough income to pay federal tax.

Taxes are unpopular. The government has no reason to "target everyone" because that's an easy way to get voted out.