I think that the links between these languages do make it easy for people from the Netherlands, Germany and Scandinavia to learn English. These countries, to give them credit, have also seen the importance of good language education, in particular English learning, for a long time.
There is also an effect which people from other countries have described which makes it harder for foreigners to learn the local language. If you approach almost anyone in the Netherlands and speak to them in semi-competent Dutch, they will often respond in fluent and nearly accentless English. This includes older people, people with only high-school education, people in official positions. So not only do you not need to learn, you are discouraged from practicing by the local population's competence and hospitality.
There is also an effect which people from other countries have described which makes it harder for foreigners to learn the local language. If you approach almost anyone in the Netherlands and speak to them in semi-competent Dutch, they will often respond in fluent and nearly accentless English. This includes older people, people with only high-school education, people in official positions. So not only do you not need to learn, you are discouraged from practicing by the local population's competence and hospitality.