Most governments have become (or perhaps they always were) a lot like publicly-traded corporations - looking for short-term gains only. Next quarterly report, next election, etc. Not looking at what happens in 50+ years. Mostly it's our own fault for electing them based on the populist-idea-of-the-day and not on what the country would look like for our grandchildren.

Amazingly it's happening not only in Europe but in the US as well, despite it being a largely successful example of a nation of immigrants (some sketchy bits of history along the way of course but still). I think those few countries that are more open to immigration will benefit in the long term at the cost of some pain in the short-to-medium term but it's an impossible idea to sell to the general public.

Having said that, taking on all refugees is more complicated than regular immigration when e.g. the host country would typically pick and choose by setting requirements for age, education, etc. And there is also the fear of encouraging even more migration by making it easier to move to wealthy countries.