Because modern optical drives and USBs are cheap; live systems already use efficient compression (SquashFS). Also, further reducing size would require removing wireless drivers or firmware, making the OS less functional.
Using 7-Zip on Windows or p7zip on Linux: kali linux highly compressed download
Instead of hunting for "highly compressed" third-party ISOs, use Kali’s own official smaller versions. Because modern optical drives and USBs are cheap;