Why are the official working hours in Singapore so long?

I do not think that its a Singapore thing, Its more about the individuals.

If you are capable and efficient, you have no issues leaving early. Nobody will be able to complain about you.

Could be many reasons, as you have alluded to. However, it can probably be summed up in 1 word:

kiasu

Perhaps this should be taken in totality with salaries, disposable income (after tax), working hours, and comparing like with like - e.g. bankers in Singapore with bankers in other countries, doctors in Singapore versus doctors in other countries. As an example, if you compare a shift-based worker (say at a restaurant) in Singapore and a shift-based worker in America, who has it better (or worse)?

Simply because its Singapore hahaha