Why don't Apple create damage resistant iPhone body so that I do not need to buy a separate case?

They can then just offer to replace the external chasis if it gets too much dents and scraches

Why would they make a durable phone? When your phone breaks you just buy another one. Apple just sold 2 phones to one person. Instead of spending more money to make their phones more durable why don't they keep making them cheap so people will buy more when they break them. It's a $1000 texting machine people you don't need it.

Then it will not so slim and sexy looking. APple also makes money from licensing accessory makers.

There's no profit (for them) in it.

Because Apple owners wet themselves over how slim their phone right befor fitting a case that makes it as thick as any budget phone! Apple doesn't do durable!

Up until last year the iPhones were more resistant by using aluminium bodies instead of glass, but wireless charging was a want for many & it doesn't work through aluminium, hence the glass bodies.