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By some estimates, the US is home to 50% of world's globally reaching corporations. Software written at those companies has giant business implications (thanks to those companies' scale), and thus the devs can be better compensated for their work.

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If this was the reason then UK developers working for US companies would be paid better

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Not neccessarily. Companies just pay each country's market wages. The difference between US and UK is that large amount of developer positions with high return on investment (for the company) pushes US market wages upwards - all those megacorps are competing for a limited pool of US people, and can afford to compete on salaries.