For Google, Twitter, and Facebook, the main reason is that there are fewer and more conservative investors in the UK. Google and Facebook in particular seemed

For Google, Twitter, and Facebook, the main reason is that there are fewer and more conservative investors in the UK. Google and Facebook in particular seemed super risky at first, and were only able to raise money from angels, even in the Valley. Microsoft is a different case. They're a generation older, and didn't rely on investors. They were in the US because the microcomputer revolution happened mainly in the US. The reasons for that are complicated, including the fact that the British government interfered with (in both the British and American senses) the British computer industry, and that getting rich was effectively banned in the UK at the time the startup was being invented in the US.