Help me understand something--why is it the tendency for game/gfx engines to become all-encompassing?

All the major game engines on the platform have support for

* 3d audio

* physics of some kind

* networking

* touch input

* accelerometer apis

Many of them have their own languages (Unity), UI subsystems, and require you to architecture everything in ridiculous ways that are non-standard on the platform.

As somebody coming from a "normal software" iOS development background, this presents me with a huge learning curve to get up-to-speed on these things. I already know how to do touch events and accelerometer handling. Apple is introducing kickass APIs for game networking and such. All I want is to draw pretty 3D things to the screen and occasionally animate them; maybe some basic collision detection.