its again demand and supply and lack of governing rules! do you really think a 1-3 years old car is discarded? mostly leased cars are replaced and previous are sold on, that can be bought for local use or export. Because a Japan is major car manufacturer, they encourage export by giving incentives and rebates.
Once, Pakistan do start making good cars by magic on one fine day, not mere assembling, people will stop importing this "used junk" 
we are not discarding junk, well as long as a car is road worthy regardless of age, it should stay on the road. We dont have yearly checks linked to car fitness but mere token payment.
Once checks are introduced and implemented, junk will be discarded when it costs more to fix the car and keep it on the road!
Road sizes in Pakistan more than enough, but no rules! though condition can be improved. e.g., living example is London, most roads are single track and narrow than most Pakistani Roads, rules are religiously followed by road users, hence far less accidents! coupled with unfit drivers and vehicles cant be on the road!
