Commercial airliners have many existing systems that'll tell you where they are. The problem is they tend not to continue working effectively 3000m below sea level after the aircraft has broken up. Neither do cheap GPS tracker systems.
The airlines' cost argument against investing in the technology is actually based on exactly the argument you're making: black boxes are usually very easy to locate.
The airlines' cost argument against investing in the technology is actually based on exactly the argument you're making: black boxes are usually very easy to locate.