A lot of them are low income and therefore recipients of the Earned Income Credit. There is a lot of fraud in those returns and the IRS sics their agents on these folks to get some fairly easy money.
The IRS goes after the most money the easiest way possible as their goal is to maximize revenue intake to the treasury. If it is going to be difficult to chase a case due to complexity or the amount in dispute is small they will often pass on the case or earmark it for later enforcement.
Note: this does not apply when they are going after a taxpayer for political purposes.
people who get the EITC don’t have any money.