Because mathematics is more about the abstract patterns and structures, rather than the meaning of individual variables (as it is for programming). More bluntl

Because mathematics is more about the abstract patterns and structures, rather than the meaning of individual variables (as it is for programming). More bluntly, it doesn’t matter what a and b are here, only the overall meaning of demorgans laws. Arguably, giving them names that are any more than minimal signifiers actually detracts from what’s important I.e. the relationship of the law is important, not the variables Bob and Sally. Personally, having the variables called bob and sally actually makes it harder, because the whole expression immediately becomes far more verbose.