They survived only to teach war to the generations of Israelites who had no firsthand knowledge of the earlier wars:
the five rulers of the Philistines, and all the Canaanites, Sidonians, and Hivites who lived in the highlands of Lebanon from Mount Baal-hermon to Lebo-hamath.
They were to be the test for Israel, to find out whether they would obey the LORD's commands, which he had made to their ancestors through Moses.
So the Israelites lived among the Canaanites, Hittites, Amorites, Perizzites, Hivites, and Jebusites.
But the Israelites intermarried with them and served their gods.
The Israelites did things that the LORD saw as evil, and they forgot the LORD their God. They served the Baals and the Asherahs.
References for Judges 3:7
The LORD became angry with Israel and gave them over to King Cushan-rishathaim of Aram-naharaim. The Israelites served Cushan-rishathaim eight years.
But then they cried out to the LORD. So the LORD raised up a deliverer for the Israelites, Othniel, Kenaz's son, Caleb's younger brother, who rescued them.
The LORD's spirit was in Othniel, and he led Israel. When he marched out for war, the LORD handed over Aram's King Cushan-rishathaim. Othniel overpowered Cushan-rishathaim,
and the land was peaceful for forty years, until Othniel, Kenaz's son, died.
The Israelites again did things that the LORD saw as evil, and the LORD put Moab's King Eglon in power over them, because they did these things that the LORD saw as evil.
References for Judges 3:12