this is from the WARN site
Illinois WARN defines notice-triggering events differently than federal WARN. Illinois WARN applies to employers with 75 or more full-time employees (excluding part-time workers) and requires employers to provide 60 days advance notice of pending plant closures or mass layoffs.
A “mass layoff” under Illinois WARN is a reduction in force at a single site of employment that is not the result of a “plant closing” and results in employment losses during any 30-day period (or, in some cases, during any 90-day period) of 25 or more full-time employees if they constitute one-third or more of full-time employees at the site, or 250 or more full-time employees.
Guessing these large companies purposely stay under the threshold to avoid having to report anything (which is smart on their part I guess)