Here is Ubisoft's Explanation for the graphical constraints on the PC version of Watch Dogs
At E3 2012, Watch Dogs looked absolutely gorgeous. On my home teleivison in 2014, Watch Dogs looks....good, but not great. Now granted, my TV only does 720p, but many have complained that even at the highest settings on both consoles and PC, the game comes nowhere near to those E3 2012 levels.
So why is that? Modders have found a way to improve the game's visual quality, but why didn't Ubisoft allow everyone to do that out of the box? Here's why:
The dev team is completely dedicated to getting the most out of each platform, so the notion that we would actively downgrade quality is contrary to everything we've set out to achieve. We test and optimize our games for each platform on which they're released, striving for the best possible quality. The PC version does indeed contain some old, unused render settings that were deactivated for a variety of reasons, including possible impacts on visual fidelity, stability, performance and overall gameplay quality. Modders are usually creative and passionate players, and while we appreciate their enthusiasm, the mod in question (which uses those old settings) subjectively enhances the game's visual fidelity in certain situations but also can have various negative impacts. Those could range from performance issues, to difficulty in reading the environment in order to appreciate the gameplay, to potentially making the game less enjoyable or even unstable.
It sounds like the development team for Watch Dogs chose stability over graphics. In theory, that's a logical move, but why not let consumers make that choice themselves?