Actually, as someone who's actually been an academic in Western Europe, I'd say the reverse is true. Certainly mathematical ability is better in Western Europe (on average). The top, no doubt, SV's top performers far exceed those Europe has (perhaps because a decent part of them are from Europe, and those weren't selected randomly. I mean to some extent that's fun because I actually know almost 10% of the top ones. From working with them before they went to the US).
But average among the population, even at universities, no doubt whatsoever, Europeans are better at math than USians. But for the top, the brain drain makes the playing field very unequal.
Besides, it's not just ability of people at universities itself. Money for programs is also very different. The US military supports US research ... and in Europe ... it's also the US military that supports research (but obviously not equally, and I must say, I find it hard to blame them for that). And in the US you regularly see academics pay for their own organizations outside of the direct academic context. Obviously that works better with better pay. There are government grants, but it doesn't compare to US university endowments (or however they sponsor that). Many of the people "brain drained" didn't leave until they were 10 years into their academic career, out of frustration of just not fighting on a level playing field just because their governments just don't support them.
If German universities paid the same, and stopped the brain drain, I have very little doubt they would exceed the output of SV. But with the current levels of people leaving, never going to happen. And to be fair, Germany's universities are nowhere near as bad as Southern Europe's universities, and even those are far ahead of most of Eastern Europe. That doesn't mean there isn't the occasional great person working there, there is, but most of the good ones leave.