I have to imagine these tables were originally created when everyone did their taxes with pencil and paper.
Asking the average taxpayer to do all the calculations for the tax bracket created the risk of too many errors. hence the IRS created the look up tables for incomes under $100,000.
That table has been $100,000 and under for at least the 40 years I've had to use them. 40 years ago, they probably covered over 90% of tax papers since incomes were so much smaller.
