I'm reminded of this famous piece by A.J.P. Taylor:
'Until August 1914 a sensible, law-abiding Englishman could pass through life and hardly notice the existence of the state, beyond the post office and the policeman. He could live where he liked and as he liked. He had no official number or identity card. He could travel abroad or leave his country for ever without a passport or any sort of official permission. He could exchange his money for any other currency without restriction or limit. He could buy goods from any country in the world on the same terms as he bought goods at home. For that matter, a foreigner could spend his life in this country without permit and without informing the police.'
It's simply a replacement of informal barriers with formal ones, plus compensation of the lowering of travel and communication costs due to technology: someone who traveled abroad in the XIXth century was either well-off so he would pass any currently existing barriers easily, or really really needed it so he would do it, too. With international air ticket worth less than a good meal, some other barrier had to be constructed or all modern nations would be quickly razed by influx of billions destitute immigrants.
> or really really needed it so he would [pass barriers easily], too
A lot of European Jews in the 1930s were trying and failing to emigrate, in a system pretty much like today's (plus the unusual bonus that their home state wanted them to leave). https://www.ushmm.org/wlc/en/article.php?ModuleId=10007455
Perhaps, but there are other options -- international bus tickets are pretty cheap. So is the ferry between, for example, Morocco and Spain. Plenty of people could afford this, or have friends/relatives in wealthy countries who could pay for them.
It is actually easier to visit some countries by air today. Visa on arrival, visa-free transit or stay—these options can be available only at airports.
Passport control as immigration regulation/deterrent definitely rings true to me, and some countries see less need for it with richer travelers.
Timisoara - Luton is a ticket I've bought many times and never for less than £20 (usual price is more like £50). Most of the times, those are clickbaity prices.
These fares do require decent flexibility -- for tourist destinations, you won't be leaving on a Friday and returning on Sunday. The £4.49 flight between London and Romania is on a Wednesday, on a Friday it's £60-70.
Ryanair use the low fares for marketing, pretty successfully. They probably expect many people to add luggage (£10-40, depending on route), or buy food on the plane (£10+) or be disorganized and end up paying more.
In Western/Northern Europe they use out-of-the-way airports -- London Stansted is 47 minutes by the fastest train from London, or over an hour by car. They have a reputation for reducing the fees they pay the airport by threatening to move to alternative airports, unless they're given a very good deal. People walk to the plane, they don't use jet bridges. The whole plane is economy class, and the seats plasticy-fake-leather, which must speed up cleaning. At airports, they'll usually use the worst (farthest) gates. All the planes are the same, so maintenance is simplified and pilots easily moved around.
As to why the US doesn't have them -- I don't know. Norwegian is a budget airline, and has fares like New York to Paris for $175 one way (Tuesday, no luggage, no food!).
The European competition authorities are quite strong. This is one part of popular opposition to Europe -- many regulations for standardization or unlocking restricted markets gore national heros in protected markets (there are plenty of other reasons for the opposition -- let's not start a flame war in this thread).
In the US this standardization process happened over a century ago thanks to the railroads. Starting in the 1980s competition law was successively weakened. In addition the cartels were able to buy legislation they needed. Hence there is little competition in airfare or internet service compared to Europe. Large chains control the downtowns of most cities and suburbs.
Explanation is extreme variability of rates. A 1000 mile return ticket in Europe can be $20 and can be $500, with average about the same as in the U.S. at around $200, i.e. 10 cents a mile. It mostly happens because most flights are between countries with extremely different living standards, and some passengers are business travellers from rich countries to poor ones, others are seasonal laborers going the other way around. There are no U.S. states with per capita GDP difference as big as U.K. vs Romania, for example.
Low-cost carrier airfares within Europe can be quite cheap. Obviously most international flights from, say, the US are going to be in at least the hundreds of dollars.
That said, I think it's worth pointing out that in 1914, the means to travel didn't exist as it does today. We've given up some freedom but made gains on capability.
But the artificial barriers we've put up are much more restrictive than the old barriers. Perhaps we should make an immigration tax, and anyone can enter if they are willing to pay $10,000. Or some arbitrarily high, but feasible, amount.
These are called investors' visas, and already exist, though the prices are much steeper: $1 million investment in a new business for an American EB-5; 2 million pounds for the British equivalent. Payment doesn't go directly to the government; instead, it's supposed to be invested in a new business that creates at least ten jobs. But there are middlemen who offer the service of actually setting up and managing the business: http://fortune.com/2014/07/24/immigration-eb-5-visa-for-sale...
'Until August 1914 a sensible, law-abiding Englishman could pass through life and hardly notice the existence of the state, beyond the post office and the policeman. He could live where he liked and as he liked. He had no official number or identity card. He could travel abroad or leave his country for ever without a passport or any sort of official permission. He could exchange his money for any other currency without restriction or limit. He could buy goods from any country in the world on the same terms as he bought goods at home. For that matter, a foreigner could spend his life in this country without permit and without informing the police.'
It's hard to believe how much we've given up.