Because so much functionality is being packed on these SoC devices now. You literally hook these chips up to some RAM and Flash and add some I/O connectors of your choice, and you're in business.

This particular SoC already has RAM built in too, if I remember correctly.

That's pretty awesome, then. Because your other options are to stack the RAM on top of the chip like older OMAP parts, or put the RAM on the board and hope you get the circuit trace timing and impedences right.

Sorry, it seems the RAM is indeed stacked on top of this chip, it's not built in.

My knowledge of SoC technology in general is fleeting.