That’s true of virtually every occupation. America is much richer than the rest of the world, and probably measures success in dollar terms more than anyone else.
A common meme today is that the median salary of Mississippi, the poorest state, is higher than the median salary in almost every other western country. Should that be of any comfort to the people who live in Mississippi, especially those below the median? Cost of living is the other half of that equation; people always leave out half the balance sheet making any such argument meaningless.
Most actors, as Wendell said, don’t make a living wage just from acting, a major reason for the actors’ - and writers’ and directors’ - strikes in recent years, trying to claw out better residuals and get guarantees they won’t be replaced by AI “actors”.
British actors, unlike, say farmers in the South Sudan, can fairly easily move to the U.S. and get jobs that pay them far more money. Shows and movies are flooded with British actors using very good American accents.
When the Internet started, pundits enthused over the “Long Tail” that would evolve, meaning that since everything would become a click away, the millions of books and songs and shows that had been overlooked would be back in the public eye and make money for their creators. That happened, but you don’t hear much about the Long Tail these days because what also happened is that the top sellers became even more gigantic and culturally dominating. Rowling and Martin and King don’t just sell well, they are billionaire fonts of media empires.
The poor salaries of 90% of the actors is not an answer to the question of “why so few episodes of British TV series?” The salaries of the top actors is a vital piece of that answer, though.