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.
We may just disagree on the size of the burden or the ability to ameliorate risks/downsides, but I'm having a hard time imagining that they cannot be addressed with _implementation_ choices on a pre-filed return.
I feel like this is important because it seems like we agree on the fundamentals: that the tax code should advantage the vulnerable, that filing should be as easy as possible, and that taxpayers should be able to discover and utilize all breaks available to them. The status quo does not seem _nearly_ good enough at these things to be worth preserving.
Things like marriage records and children aren't currently kept private such that the Child Tax Credit and Joint Filing rules couldn't be automatically applied. It's not immediately apparent to me that vulnerable populations would necessarily be at a disadvantage.
(Basically, I figure that if I'm high-income, it's my problem to sort out my taxes to my greatest advantage, but it should be easy for folks who don't have the luxury of time or money.)