The IRS estimates 21-26% of all EITC claims are improper. The rules are quite complex, which results in more errors and it is relatively straightforward for the IRS to detect certain classes of mistake or fraud through automation (i.e. to detect if the same child was claimed as a dependent by two different people on their return).
Congress made the EITC complex. EITC errors/fraud cost the public purse $10-20bn a year. Is the IRS just supposed to ignore that?