Yesterday I experienced for the first time an under-carriage camera, displayed on the in-flight entertainment system. We got to watch the runway on takeoff, and for about 5 minutes after, and landing, and about 5 minutes before.
I must admit, while it was a bit scary (especially since the camera wasn't quite centered, leaving the impression we weren't approaching the runway head-on), it was also entertaining, and offered a much better view than even the window seats offer.
Why isn't this feature common? Is there any (known) official reason? Or has it just not caught on?