We're not talking about "high-income" people. The biggest tax benefit for low-income people is the Earned Income Tax credit, which is calculated based on number of dependent children. Then there are all sorts of middle-class credits and deductions even for people who do not itemize: child credits, child care credits, child care deductions, teaching supply deductions, etc. And large numbers of middle class people use the mortgage interest deduction.

And even relatively high income people ($100,000+, which might just be a household of two teachers) might be hesitant to dispute a "bill" sent by the IRS.

As to applying these deductions automatically: the IRS simply lacks the information. There is simply no database (federal or state) tracking which children live with which adults (the credits are computed based on dependent children, not biological or legal children). There is no federal database of marriages, and while there are marriage certificates at the state level, it's unclear whether there are any easily accessible databases of married couples. There are no databases tracking which daycare you send your children to, what you spend in college tuition, etc.