Because they take the environmental impact of producing things. This is one of the major factors, plus china has the most abundant supply of rare earth metals, most are mined in China so it makes no sense to export them to produce elsewhere.

You've also got labour costs, aside from wages (no idea what Chinese wages are like in a factory) but in the UK for instance an employer has to pay for pensions, taxes on the site, national insurance contribution on the employee itself.

What you could do at work, is look at your salary, and then ask your HR team what the actual cost is per worker. A fair amount of employers will have a typical cost per desk or workstation which will be substantially higher than you expect.

My old company was an insurance broker based in London, the average cost per seat as it were was around £65k but the mean wage was in the 30s.