Amsterdam hosts a major internet exchange. It's not a bad place to build a datacenter and there are many. Northern latitude brings free air cooling, but also additional distance to clients. Lots of peers in AMS-IX, but not a lot of oceananic cable landings (one with two paths to the US, but most of the submarine cables land nearby in Europe)

Whether it's generally a reasonable place to build them isn't the percentage. The number seems to be ~3%.