The border that now exists between Papua New Guinea and Indonesia is a legacy of the Dutch decision in 1828 to annex the western portion of the island. Back then, they simply drew the line at the 141st longitude. Following that, the Germans and the British grabbed the north-eastern and south-eastern portions of the island respectively.

The border is largely straight, north-south. But that isn't unique or even unusual. The Australian colonies around the same time were defined by straight lines on the map (largely because the landmass was not surveyed in detail) and it is extremely likely there was no detailed information about the PNG landmass in 1828 when the Dutch created the border.

A quick check of the world map shows many straight lined borders - especially in Africa and North America.