I speculate that Ford has never offered a V8 in the Ranger for a couple of reasons...
1) Small pickups have a very small profit margin. They are expensive to build, but can't be sold for very high prices. Upgrades like the V8 would drive the price up beyond what people will pay. This is the same reason why Rangers don't have things like power seats.
2) It would take away 1/2 ton sales. To Ford and GM, their 1/2 ton trucks are the most important vehicles in their lineup. They are the biggest sellers by far, and claiming the #1 sales title is important for advertising. A small V8 truck would eat into sales of the big trucks.
3) Low demand. Economy is a big selling point for small pickups and really the main reason why they exist at all. Small pickups were not really seen in the US until the cost of gas became a big concern to people. A V8 Ranger would only appeal to a small number of small truck buyers.

The good news is that Ford found it worth their while to stuff 5.0s in Explorers and swapping is fairly straight forward.