I lost my iphone some time back so when I called AT&T to report it, they kept on insisting me - please wait for few days as you may find it. They also told me once we report the phone as lost and block it using IMEI then this change can't be undone incase you find your phone then it can't be activated again. And, the block using IMEI does not work across carriers which means if its blocked in AT&T then the person carrying that lost phone can activate the phone on any other network like verizon, spring, t-mobile etc.

Carriers can maintain a centralised database to keep list of stolen phones and can also undo the change incase the owner finds it. They can also track the people who are calling using stolen phones but they dont do it. The best reason I can guess for not doing that is as you said - why they will do something which will hurt their own business