This is a really good question and one that is rarely asked, and people dismissively say "because they want to screw us!" But then, why can't international airlines come in and compete with American airline companies, which would then cause the US Airlines, JetBlues, and others, to start competing...? Well, because they're not allowed to be law.
Non-American airline companies cannot provide domestic flights in the US because of 'cabotage' restrictions, an ancient word, but the law of the land. It's a protectionist law but it also prevents a company like Emirates from coming in and flying vastly cheaper flights using vastly lesser paid employees, getting gas subsidies from the UAE, and paying no corporate taxes in the UAE. If Emirates came in and competed directly with Delta, Delta would be put out of business.
But beyond that, the flying experience is further made worse in the US By a few major factors:
- There is very little competition between country-wide air travel. You have 3 major country-wide airlines, and then mostly only regional airlines, but because of individual contracts at specific airports, one airline will "dominate" an airport and consider it their "hub."
- Non-US airlines have wages far below US airlines, which are protected by federal laws and importantly a very strong Airline employees union.
- While Singapore or Emirates can hire only beautiful, twenty-something women, and mandate that they wear a specific outfit, wear certain makeup, and act a certain way, American airline companies are forbidden from discrimination in hiring along any race, sex, age, or gender lines. Singapore Air only hires beautiful young women for customer-facing positions who are only temporary, contract employees and only the best of the best (or most beautiful) get their contracts renewed.
- None of the US passenger flight companies are state owned or state subsidized, which isn't the case for dozens of major airlines that they compete against.
- Airline jobs are high-paying in the US and are considered "career" jobs. THis is a good thing for airline employees, but it can have mixed results for passengers, who may feel like trips to the airport are more like trips to the DMV... Airlines in other countries that generally perform highly in these comparisons are aggressive with hiring and firing, employ many workers on contractual basis, and have wages well below your average American airline employee lifer... Who may be in the industry for decades, have seniority over routes, and may not be as motivated as a young person competing for a contract. This isn't the case for all European airline companies, though it specifically is the case for Cathay, Singapore, Emirates, and many other high performing airlines.
- Geography and population density play a significant role in European air travel, and airlines also have to compete with a functioning rail system between many large cities. This drives ticket prices down. Further, there isn't government & airline collusion in Europe to limit flight numbers like there is in the US. I don't know if you remember how air travel used to be, but in the 1990s, you'd regularly fly in empty 747s in long trips... This is virtually eliminated now, and if you have an empty seat next to you on a flight, it's considered a miracle. Europe hasn't adopted this model as quickly. This leads to more flights that need seats filled at lower costs in Europe.