Why didn’t God destroy the Canaanites himself, rather than ask Israel to do it?

There’s no question that God could’ve destroyed the Canaanites directly. Some of the methods by which God destroyed people directly in the Bible:

• The ground swallowed them (Num 16:31-32)
• Plague (Exodus 32 : 35)
• Snakes (Num 21: 6)
• Kill directly (Num 8:17, Acts 5:5-10)

But he chose not to do this to the Canaanites, in order to test the Israelites:

“I will no longer drive out before them any of the nations that Joshua left when he died, in order to test Israel by them, whether they will take care to walk in the way of the LORD as their fathers did, or not.” So the LORD left those nations, not driving them out quickly, and he did not give them into the hand of Joshua. (Jdg 2:21-23)

They failed the test too:

So the people of Israel lived among the Canaanites, the Hittites, the Amorites, the Perizzites, the Hivites, and the Jebusites. And their daughters they took to themselves for wives, and their own daughters they gave to their sons, and they served their gods. And the people of Israel did what was evil in the sight of the LORD. They forgot the LORD their God and served the Baals and the Asheroth. (Jdg 3:5-7)