Fixed gear chains don't need to be particularly strong. Shimano once made a 10mm pitch track chain that must have been 30% weaker than a standard chain, and it was used at a world championship. Derailer chains break because there can be a lot of sideways force when they engage 2 cogs at the same time during shifting, and because they are difficult to join. Fixed chains are never out of alignment, and there's less chain tension because you don't use low gears. If you use a wider chain designed for 7 or fewer cogs they are easy to join, and cheaper too. This is one of the few cases where cheaper actually works better.

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