I'd argue that's because game development is not a CS discipline. Game tech / engine is, but games need more man-hours in artistic disciplines like modeling, animation, level-design, story-telling etc. than they need for cs.
There is a lot of 'game middleware' out there for the CS part of the equation. Game engines themselves are not monoliths, using 20+ middleware components is common.
The game design part is done using a monolithic interface to the technology (the engine), but that's because that job is not 'programming'.