In any sentence if you ask yourself what are you talking about? Then that is the object. Here, you are talking about money. Hence that is object. So How much he earns .. means, What is the amount he earns? Another example. Jack is throwing stones. Here ask yourself What is Jack throwing?? Answer: Stones, so Stones is the object!! Hope you got it :) :)

Why shouldn't it be used as an object? Could you clarify your question?

How much(Money) do you think he earns every month?

MONEY (as earned) is a non-countable noun, hence used "how much?" If we refer to a particular of bills for instance, we use "how many?" (ex: 'how many $100 bill does he earns in a month?). :)