It all comes down to U.S. tech companies being much more successful, mainly because of the scale of the U.S. market. Any U.S. startup can immediately address a market of $21T (GDP) without any regulatory overhead or language considerations. This causes engineers to be insanely productive in terms of economic productivity. In comparison, Germany, the largest country in the EU, has a GDP of only $3.8T. It's much harder to scale across the entire EU, and even then, the entire EU market is still smaller at $15T.
The second question is, why do international companies like FAANG underpay their European employees compared to their American peers? The reason for that is the prevailing market wage. Any company will pay as little they can while still getting the caliber of talent they need. Given the lower economic productivity of engineers working in local tech companies, the prevailing market wages are lower, so FAANG don't need to pay as much to get the top talent.
The second question is, why do international companies like FAANG underpay their European employees compared to their American peers? The reason for that is the prevailing market wage. Any company will pay as little they can while still getting the caliber of talent they need. Given the lower economic productivity of engineers working in local tech companies, the prevailing market wages are lower, so FAANG don't need to pay as much to get the top talent.