I don't think that limit exists for technical reasons, I believe that it's an arbitrary limitation applied by Intel/MS businesspeople trying to protect their higher profit product lines.
Indeed, and I was specifically referring to the Atom line of processors and how Intel has crippled them (no ECC, 64 bit capable but <= 4 GB max physical memory ... although I must note both of those would increase their cost) and otherwise not invested sufficiently into them (worse than subpar for the market chipsets and not even trying in the SoC direction, the latter more of a cultural and business model thing).