35mm film is more square than rectangle. When you shoot wider aspect ratios, the whole frame is exposed. The eyepiece on the camera has lines on them to allow the DP to see the framing for the desired aspect ratio vs the whole frame. So it wasn't just a Titanic as example, it was pretty much all film was shot like that.

When digital cameras like the ones from Red came out, you can tell it the aspect ratio so it only saves the active pixels of the full sensor and ignore all of the out of aspect pixels. That's a brave operator doing that, and I've only seen it in the wild once.