Italy currently has a slow growing economy and needs a large number of younger economic migrants to increase Italy's competitiveness and pay taxes.

I wouldn't. Being in high demand in Silicon Valley is amazing even compared to Seattle. I wouldn't move to a backwater for fear of having to move again every 3-5 years.

Huh?

Also, those places are way more international than most places in Europe, which makes relocation more attractive. Good luck attracting non-Italians to Italy, or non-Polish to Poland.

I would gladly move to Italy with its amazing culture. I would not move to a place so diverse and full of recent immigrants that it no longer has any specific culture. Am I unique here?

And yes, people relocate to Poland a lot, from ex-USSR countries for example.

I'm pretty sure both London and Switzerland have cultures (OK, UK culture is mostly just drinking but still...).

Large software hubs generally need to pull in people from around the world, not just USSR. And that's the crux of the problem. London, Berlin, Paris, Zurich are all major international cities - it's easier for people from all around the world to settle down there than in places like Warsaw or Perugia.

You are aware that the Soviet era is long ago and the youngest people educated under a Soviet system are mid-30 by now? There is plenty of universities in Poland teaching among other things also Computer Science.