Because uncontrolled mass migration brings cultural and societal changes that host nations' indigenous populations reject. The nature of immigration has changed over the last 20 years, from migrants that wanted to work hard, integrate into the host country and work for the good of the host society. All you see now are migrants that do not want to integrate, do not want to work, want the benefits, increase crime levels and have no intention of becoming part of their host societies. These facts are borne out by the opposition that is seen amongst the voting citizens of Europe, it is not a mirage.
Those citizens do not care a jot for the economic benefit arguments put forward by the Globalist leaders, all trust has been lost in the political class of the European Union, economics are a side show (although in countries such as Greece and Italy it is high on the agenda), economic arguments are not believed. The priorities of the citizens is an anti-migrant one.
So we have a political class that bases arguments on economic grounds that the citizen class does not believe and a citizen class that bases arguments on cultural and society arguments that the political class ignore. The question is who will win, it appears the political class are on the back foot and have no intention of changing, I wonder where this will end up.
I understand that you guys naturally like to talk trade and economics here, but to do so is to ignore the real issues.
If Theresa May goes down the political trade route over Brexit, there will be hell to pay for the Tories at the next general election in 2021, this will let Labour into power with their currently extreme left wing ideology, a nightmare scenario for the UK.