Why NFS Couldn't Just "Reuse the physics from Underground": A Speculation from Your Local Car-Loving Underaged Weeb.

As it says, the logo was created by http://gyroxopex.deviantart.com/

To begin, let me tell you that if they could have just reused the engine, they definitely would have done it. Reusing an engine is cheaper and easier than making a new one. On top of that, they already know how to use it and have plenty of assets for it. However, Carbon was probably the absolute limits of what the engine could do. So, they had to change.

And change, they did. Prostreet: The first in a line of games with janky physics.

So, they tried with Prostreet, but had to tweak the engine for the individual game modes, which was iffy at best. But, think about the difference in the amount of content between Prostreet and Carbon. They probably had to make the new engine because the previous engine couldn’t be ported to 64bit, and with the limitations of 32bit, they couldn’t fit as much content in. Attempting to listen to the fans, they tried to make another game like Carbon and Most Wanted.

Need for Speed: Undercover

Like Sonic ‘06 they tried to give what the fans wanted so badly, but couldn’t get the engine to work like the old one. But, the new engine allowed them to make the Tri-City Bay the largest open world environment we’ve seen in an NFS game at the time of its release. However, after the game flopped, they scrapped what they had, and to keep with EA’s yearly release schedule, they decided to start outsourcing full games.

Enter Slightly Mad Studios

The release of a new Need for Speed game not made by Black Box started a new era, in which Black Box, Criterion and Slightly Mad Studios would pass the baton of the Need for Speed name around to keep up with that yearly release EA wanted. With Ghost games now being the sole developers of the Need for Speed franchise, and likely not having the original creators of the NFS Underground/Most Wanted engines, the source code for the engines, or a way of properly porting it to the modern day, they have to make do with what they have, and if nothing else, they’re trying their best.