Because humans suck at serialized content.

7 +- 2. [1] That's the number of things our prefrontal cortex/short term memory can track at once. That's why we (humans) organize things into hierarchies. That's why the best team size is around that number. Etcetera.

Heck, everything in the world on a computer is serialized into memory or onto disk. Or addressed as some disk in a serial array of disks. Serialized as in, "there's some data somewhere in these 2TB that tell me where in the same 2TB the rest of the data is." Computers excel at this. Humans are terrible at this.

I guess my point is, humans are the reasons we need modules.

[1] http://en.wikipedia.org/wiki/The_Magical_Number_Seven,_Plus_...