Because it's harder to do it in a complicated formal way than simply do it in the most straightforward way. If a type of mistake happens only very rarely and is typically easy to detect, it's almost certainly not a good idea to worry about it. You pay each time you express something using the more complicated formalism, and each time you work on parts that somehow have to deal with the more complicated artifact, and only every Nth time there is a marginal gain from using it.