First of all, it requires some study and serious effort.
Here is some direct and fascinating evidence from Vox Day, a former libertarian personality who is now an Alt Right blogger, about why he turned against libertarian ideology both on the issues of immigration and free trade:
As we can see, it was the feeble libertarian responses to the problem of free movement of people, and gaping holes in the theory of free trade that did it for Vox Day. Vox Day also cites Ian Fletcher’s Free Trade Doesn’t Work: What Should Replace It and Why (2nd edn.; 2011) as an influential book against free trade.
Libertarianism is flawed by its insistence that the individual is sovereign, and its inability to see the importance and interests of larger groups, above all, the family and nation. Without government policies to secure and create a viable political unit of people with common interests where those people can flourish, individuals cannot flourish.
What would happen to America if it went full anarcho-capitalist, and if its government were abolished and government control of the borders were ended?
As all barriers to trade and capital movement were removed, the collapse of manufacturing and industry would be accelerated as free trade under absolute advantage would implode the US economy.
As centralised government border control was abolished, and decisions on immigration flows were totally privatised and decentralised, there would be a tidal wave of mass immigration from the Third World, which would destroy the demographic and cultural cohesion of America.
The owners of any big business and industry left would also happily bring in millions of cheap, foreign and easily exploitable labour from the Third World to smash wages and labour rights and make themselves internationally competitive.
Given the fact that there would be no US national government concerned with national security, hostile foreign governments like China or the Arab Gulf States would be able to buy up vast real estate, property and national assets, and then import millions of their own people, which would reduce vast areas of America to colonies of China or Saudi Arabia. As in Europe, whole areas would be gradually lost to segregated and fundamentalist Islamic communities, deeply hostile to the culture around them.
The result would be increasingly isolated, militarised gated communities of wealthy Americans, surrounded by a sea of Third World poverty, economic collapse, crime, drug cartels, violence, social collapse, intercommunal violence, and then civil war.
In short, America would turn into Brazil.
Some libertarians have realised this. Others remain mired in their utopian cult, as cult-like in its own way as dogmatic Marxism/Communism.