It seems that you have a bit of a misconception on what the "Vikings" were. There was no single unified culture in Scandinavia in the Viking Age to point to. There's a cluster of similar cultures all across the area, connected by the sea. The Norse word víkingr, in fact, refers to a sea-raider of any type, not specifically what we would identify as a Viking. Irish pirates are identified as víkingar just as surely as Danish raiders. The Vikings were instead groups from numerous small kingdoms and tribes who were independently seeking to accumulate wealth and power and gobble up their neighbors. So it's not unreasonable that a bunch of coastal petty kingdoms would all end up as coastal raiders, while that didn't happen farther inland.

As to why peoples from modern Scandinavia wound up going raiding by sea, they were already very comfortable with boats as the best way to move trade goods around Scandinavia. Ship travel along the Scandinavian coast was relatively easy and safe, especially compared to the dense forests of Sweden and Germany, so arcaeological evidence reveals complex networks of trade across Scandinavia from the Bronze Age.

This cluster of linguistically and culturally related people in Scandinavia did have close cultural neighbors in the Saxons and the Frisians and the Goths, and millenia of contact and trade by the Viking Age. The ties are definitely there between Denmark and Northern Germany. And indeed, the "Saxon Invasions" of the 6th and 7th centuries had Frisians, Angles, Saxons and Jutes (i.e. people in modern Denmark) traveling by ship to the British Isles. But, they are farther inland, and with how important ship travel was in Scandinavia and the Baltic, it wound up leaving the Saxons more culturally distinct from the Danes than the Danes were from the Gautar and the Swedes by the 8th century. And then, of course, Charlemagne's conquest of the Saxons ensured that they were culturally tied to Christian Francia in the Viking Age, not pagan Scandinavia.