Because it's technically not "audit". They just send you an automated letter if they find any discrepancies: https://www.irs.gov/individuals/understanding-your-cp2000-se...

And they _will_ find them. For example, one year I forgot to add a line from one of 1040 forms to my return. I got a notification from the IRS about a year later that I have under-reported taxes.

And with the defunding of the IRS, they'll severely limit the complex audits.