It would be trivial to keep track the IMEI of the phones and to render them worthless by blacklisting them on the mobile networks.
Why is this not done?
Same applies to any stolen phones btw.
Same applies to any stolen phones btw.
It's extra work for the carrier and can result in innocent people being denied service so they lean towards not banning IMEIs.
Also it can be spoofed so it isn't a surefire way to ban these devices.