Why are 3D Renderers slower than 3D games?

The reason why is that 3D renderers use a technique called Ray Tracing. This is where it plots the path of every single ray of light as it bounces off everything from every light source. Also Spline based editors result in an infinite number of potential surfaces, which can make the calculations of the collisions slow.

This is very slow, but delivers the best quality image.

Games use a technique called Rasterization, this is a simple conversion of 3D coordinates onto a 2D surface. While very quick, and quite good looking (With the aid of shaders)

There are lots of effects that can't be done using Rasterization that a ray-tracer can do. Especially when dealing with perfect reflection and refraction, as well as techniques such as radiosity, which works out how the color of light reflecting off a surface affects other surfaces around it, which a rasterizer can't do.