Why is it that US Presidential candidates do not issue formal written manifestos of their intentions if elected - as British political parties are required by custom to do? 

Manifestos are important in Britain. Inexplicitly and loosely worded as they tend to be they nonetheless provide some level of formal announcement of what a government in office holds a mandate to do, and perhaps more importantly, not do.

They also require the party concerned to be formally specific about their intentions. The [Conservative manifesto of 2015](https://s3-eu-west-1.amazonaws.com/manifesto2015/ConservativeManifesto2015.pdf) ran to 85 pages. 

It seems that presidential candidates, like Trump, have been able to get away with quite vague expressions, e.g. about a future relationship with Russia, or building of a Great Wall of Mexico. Had he been required to state his policies in writing, it might have been rather more difficult for him to avoid his utterances when in office. 

And one can learn a lot from what parties put in and do not put in their manifestos. For example, following the huge fanfare that the Conservatives have recently been making about the restoration of grammar schools, we now discover that their intended manifesto commitment will be to establishing just 20 such schools. i.e. they are only talking about 0.6% of the number of Secondary Schools in the country!