The reason is because of the cost and hassle of getting all needed parts together to perform the swap. It is POSSIBLE... just easier to buy the whole 4V engine.
You would need new heads, timing covers, exhaust manifolds, intake manifolds, throttle body, ECU, injectors, and more.
In addition, you still wouldn't have the 4V forged crank and your compression would go way down dropping your power.
If you want a 4v- just buy a used one out of a wrecked cobra or as a crate engine.