[hackcasual]

Because 2 FPGA cores don't give you the same bang for your computational buck as 2/4 general purpose cores. You're better off hanging an FPGA off a fast internal bus with an expansion card, rather than try and cram an FPGA on a CPU die.

Think of them like graphics cards, but even more niche. Trying to stick them directly into the CPU isn't going to provide the power of a dedicated add on.

[guiriduro]

Although if they are on-die, you can benefit from shared L2/L3 cache, and lower power/increased performance of the CPU-FPGA coupling, shared memory path (lower cost than dedicated, although they can compete/starve each other if there isn't good synergy at the OS level.)

[hackcasual]

Yeah, the problem is any FPGA solution that integrates directly with the CPU cache is going to be a bit underpowered due to fitting on the silicon. Even the integrated CPU/FPGA SoCs I've seen have the ARM core separated by an interconnect