Firstly because most do not think in the terms of the future; seeing these kinds of trends and making logical deductions from them is not something that comes naturally to all people.
Secondly, many have been brainwashed into thinking that it's "racist" and somehow immoral to want to defend your own culture against foreigners so that your children have a future. These beliefs are crippling and pathological, I classify them as a form of self hatred. It *makes no sense* to allow competitors into your midst and to multiply. How do you benefit from this? Generally the most people can tell me is "more ethnic restaurants"
I am wary about diverting this thread though, as it's not about immigration, so this will be my last comment. If someone wants to create a new thread about this topic, then we can continue this discussion there.