A simple answer:

Power.

A slightly less simple answer:

Power, and synchronization reliability.

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Time-of-flight active range finding methods require you to project a signal out into the field, and to do so with something powerful enough to reflect back to the recording device with enough signal strength left to get useful readings off of.

That eats battery power, and while it may be useful in very low light or other challenging conditions, that is not a huge part of typical photography.

The other part is that knowing how far away something is from the camera doesn't actually do a lot for you when it comes to focusing a lens. In a carefully calibrated system it can be used in a round-about way to achieve focus, but it does not actually tell you anything about the focus.

So you can know that something is exactly 10.1543m away from the camera, and you can set the lens to the focus point that is was told should work for 10.1543m away, but this in and of itself doesn't tell you anything about whether or not the object is actually in focus... Since such a system does not rely on the focus of light through the image taking lens, then it has no means to confirm what the state of focus actually is.

Instead, camera manufactures have relied on systems that tie in closer to existing focusing methods. Canon has a 'focus aid' system built into many of their external flash units, which can project an IR grid into the scene to help the standard focus system lock on in low light for example. This grid is picked up by the standard optical focus system in the camera, and is based on how the lens is focusing light.

• Uses existing tech already in the camera.
• Uses power only on demand, assuming you remember to turn it off...