Why can't I find SOCs with integrated memory?
Some FAE explained this to me a few years back like this:

The process for fabricating DDR memory is a completely different one than the one used for the processing unit. Also, the average life time for a DDR2 IC or DDR3 IC or whatever DDR IC is much shorter (~ 3-5 years) than the availability for the processor (10-15 years). So even if they build this package-on-package IC they couldn't guarantee the lifetime, thus reducing their marketing point.

I know what you mean, laying out the DDR2/DDR3 interface is an error-prone task and requires some very low level software digging - I personally do not like that as well.