Standard German is a written language that was artificially created. Its roots are High German(in the south) and Middle German(well in the middle). Low German(in the North) wasn't really seen as German at all and it was really looked down upon. Because of that there is almost no Low German influence in Standard German which is why a German speaking Low German will sound to most foreigners like a Dutch.
I've heard there's a dialect continuum between German and Dutch among rural northwestern Low German dialects.