Why don’t DSLRs use laser rangefinders for autofocus?

why not include a laser rangefinder in DSLRs? This would let the camera focus in the worst possible conditions when the standard methods fail.

This is really a question that calls for some speculation, but I can think of a few reasons:

1. Focus points. DSLRs typically have multiple focus points that let the photographer choose which part of the image should be in focus. A laser rangefinder would probably only support one point, so it would be much more limited than what existing AF systems provide.

2. Cost. Sure, the laser diode found in a laser rangefinder might be relatively cheap, but the electronics needed to detect and time the round trip of a pulse of light also come at some cost.

3. Calibration. Existing DSLR AF systems don't really care how far away the subject is, they only care about whether the subject is in or out of focus, and in the latter case which direction to adjust focus. A laser rangefinder measures actual distance, but getting the lens to focus precisely at that distance would require some degree of calibration, and that would need to be repeated for each lens the photographer might use.

4. Need. It sounds like you intend for the rangefinder to work as a backup system, not a replacement for the existing AF technology, but it's not clear that current AF systems fail often enough to require a backup. In cases where there's a problem, other aids (the AF assist light built into many bodies and also speedlights) already help.

5. Optics. Laser rangefinders have their own lenses built-in; it might be tricky to build one into a camera body in such a way that it works reliably with interchangeable lenses, and without interfering with the existing AF system, the reflex mirror, or the image sensor.

6. Marketing. The lasers used in rangefinders may be safe for the eye, but that doesn't mean people will necessarily feel comfortable having one pointed at their eyes.

The answer to most questions of the form Why doesn't product X include feature Y? is that the feature in question doesn't provide enough benefit to justify the cost. The points above are really just some suggestions for reasons that building a laser rangefinder into a DSLR might not make economic sense. Basically, it comes down to adding a bunch of complexity and cost to provide a feature that's not really needed.