Using multi-core GPUs in an SOC

Power consumption and heat may be factors, also cost then. Maybe it's cheaper to do an APU or other GPU in small builds vs a PVR/ARM SoC.

because it's harder to pull off and get right

>>59298353
Because the industry hates change, and risks.

Bro amd APU's are basically exactly this, including those in the xbone and ps4. They're a combination of weak CPU cores and a cluster (I believe somewhere in the HD7850 ballpark if memory serves) of GCN GPU "cores," or Compute Units as AMD calls em. The chips also share a pool of GDDR5 memory between the GPU and CPU portions respectively.

I couldn't comment on how similar or dissimilar this is to a traditional desktop, but the slew of terrible console ports suggests it's far from perfectly seamless.

A better question is how realistic is an ARM variant in this configuration? The answer is unless AMD does it, not very. The desktop grade CPU and GPU markets aren't just hard to get in because of stiff competition, Intel, Nvidia, and AMD hold countless patents on the technologies that define these devices from which industry trends and standards are based. I point to AMD because they have the groundwork in both camps laid out already and they've mentioned a passing interest in ARM in the past.

Furthermore, ARM's strengths lie namely in the mobile market. In that area, the "multicore" arrangement familiar to desktop GPU's of today might be highly inefficient much like x86 is. An approach proven effective in one application doesn't exactly suggest universal capability.

Tl;dr it's probably not worth it to make such a device to begin with, let alone code for it.