Because Israel

Numbers 21:24-26
And Israel put him to the sword and took possession of his land, from the Arnon to the Jabbok--but only up to the border of the Ammonites, because it was fortified. . . .

Proverbs 19:5, 9
A false witness will not go unpunished, and one who utters lies will not escape. . . .

from Arnon.

Genesis 32:22
During the night Jacob got up and took his two wives, his two maidservants, and his eleven sons, and crossed the ford of Jabbok.

Deuteronomy 2:37
But you did not go near the land of the Ammonites, or the land along the banks of the Jabbok River, or the cities of the hill country, or any place that the LORD our God had forbidden.

Deuteronomy 3:16
and to the Reubenites and Gadites I gave the territory from Gilead to the Arnon Valley (the middle of the valley was the border) and up to the Jabbok River, the border of the Ammonites.