Because hard work is referred to something general. It is hard work......It is a hard work, does not make sense in English. In English we would not say....'it is a work' we say, 'it is work' the rule is the same if you say 'hard work' If you want to refer to a specific type of work, you might say the word 'job' Ex. It is a hard job. It's a hard job. Hope that helps.

Because 'work' is an uncountable noun. Job is a countable noun - it is discrete. You can do one job, and when you have finished that - you can do another one. But all the time - you are doing work. It is a bit like water. You can't have 'a water' (ok - except if you are abbreviating 'a glass of water', or 'a watering')

In most cases, "job" is a count noun, and "work" is a non-count noun. However, it is also the case that almost every single non-count noun can be a count noun in some contexts. For example, if "work" referred to a "work of art," than it would be a count noun.