Why has the US always been a conservative country with regard to immigration & flexibility of who they let in via employment?

Namely only the global elite.

This will hopefully be a thought provoking question here, as I'm not a US Citizen, but dual Dutch and British with hence a different world view and also a very strong supporter of the EU.

NAFTA never wanted to go down the same road as the EU, where a German can legally live, work and take any job in France, and an Italian can legally live, work and take any job in Belgium.Why?

Similarly, apart from Camp America for three months and unlike the two year working holiday visa in Australia, the US also also never adopted a similar scheme for Europeans to come over, take a secession of bar or Starbucks Coffee jobs and generally backpack and travel round on the Amtrak and Greyhound over two years to see all 50 states. Why? I see no harm in such a cultural experience, and for most Europeans its only 90 days at a time on the visa waiver.

Finally, what happens if Nisha, a Receptionist in the UK with Ford, would like to experience the same role working at global HQ in Detroit? She can't and is locked out from having that experience, when the company could just do an international job share swap and send someone over from the US instead, who also wants to see the UK to fill her role so 'no local is done out of a job' so to speak.

We're meant to be living in a highly globalised & fully integrated world economy, so why the continuing restrictions, tightness & lack of flexibility over the free flow movement of labour between US & Europe, both 1st world?

What do you think you accomplish by pretending to be different nationalities.

Immigration policy is a mess, because there are too many people influencing the policies. Working class resentment would ban all immigration - working class hostility to poor immigrants is very American and never goes away. Business interests want a steady supply of cheap labor, and in the other direction want a ready supply of highly skilled workers so they don't have to bother training native workers. If your friend was a nurse, she could probably get in without any problem (the turnover in nursing is very high).

What the US does suits them and their country, they don't set immigration laws to suit you and your obsessions about serving coffee

The US has let more people immigrate than the Dutch and British combined

That's just patently untrue. Historically the US has had the lowest bar for immigration of any nation, taking in some millions during the 19th and early 20th Centuries whose only litmus test was that they not be carrying a communicable disease. But clearly as the population grew and as automation meant even manufacturing jobs required some basic education the nation shifted its policies to favor those with only needed skill sets (like every other country has done). The EU is an economic zone. NAFTA is merely a trade deal. Two entirely different kinds of arrangements. The EU includes only nations with similar standards of living, attempting to achieve parity. NAFTA was created in order to exploit the cheap labor of a developing country and take advantage of Canada's national health scheme (meaning US jobs sent there don't require expensive medical benefits). "Nisha" would be an idiot to want to leave the UK for Detroit. But more to the point, "LaNisha", a Detroit native who lives in a city with a nearly 10% unemployment rate needs the job worse (as it's her only hope of gaining the benefits that allow her to take her kids to a doctor). The US already offshores most of its low skilled jobs and imports illegals to do those that can't be exported. This landscape is unfair to US workers and what you're suggesting would make an already bad situation far worse. Your schemes to import (mostly unwilling) European workers for unskilled labor in the US entirely misses the fact that the US has the most generational poverty of any nation in the G8. We taxpayers would rather Ford pay LaNisha to work so that we don't have to pay her to sit home doing nothing. Nisha, on the other hand, already has a gig (and free health insurance) in the UK.

Your opening sentence is false.
Historically the US has had the most liberal immigration policy of any country in the developed world.
Even today its much easier for a European to move the the US then it is for an US citizen to move to any EU country

The US has no need for receptionists.