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Because we need young people to pay our pensions when we're retired.
And that right there, is the main reason why the UK government has encouraged high immigration levels. Migrants are usually young net contributors to the economy. When you've got an ageing population with expensive long term conditions and a multi trillion pound pension deficit, encouraging working age people to your country and in turn hoping they have kids (who then spend their lives working and contributing as citizens), is much easier than getting your native population to have more children.
Either that or fuck the plebs and change the retirement age to 80.
Because we need young people to pay our pensions when we're retired.