Because renting a half decent flat (new build, 60sqm, inner London) costs 2000£ per month, good food is extremely expensive (Waitrose would be an average supermarket in Southern Europe), the NHS is not as good as its continental homologues, so you have to pay for healthcare, public transport is very expensive, you have to deal with the immigration office (which western professionals aren’t used to), etc…

The cheapest nursery in inner London cost more than 1600£ per month, so if you have a child you’ll need 4000-4500£ per month (or 75-80K per annum) just to put a roof over your head. If you want to live like a professional or save some money, you’ll have to multiply that by 2.