Because larger populations create
A need for more hospitals
More doctors
More police
More sewage
More cars
More lorries
More pot holes
More social services
More crimes
More police...........
You get the general idea
Plus some people are more adverse to the extra risks of importing 'trouble'
Add in the xenophobic or the good old racist element and there you go.
All the above have been discussed quite often. Its not that much of a surprise its discussed either here or in other countries.
Governments are painfully slow at reacting to change so you are forever playing catch up through population increase, and there is the cost. Throw in major economic knock backs like the economy crashing, covid, shit govts and the biggest of all in the history of mankind and probably the universe Brexit which according to estimates released is 10 trillion pounds per day.
At least :-)