Why belts...?

[jayemm89]

Watching a part of an old documentary on Youtube, regarding the Cosworth DFV. I noticed that it was entirely gear driven. This got me thinking, I know a few cars which are chain driven (i think all of my BMWs have been) but why is it that most engines use cambelts for timings, rather than gears? Is it a cost thing, or is there a benefit for a non-racing engine of having a belt?

Google didn't seem to turn up anything helpful with my searches. Pretty sure there will be a good reason, just curious what it is.