Video processing is generally a parallel bandwidth-heavy task with predictable memory access patterns whereas compilation is NOT. Thus video processing lends well to hardware implementations. Additionally everything h.265/VP8 or later was designed with people from HW companies on the committees and thus the algorithms were adjusted specifically to benefit HW impl. Compilation is extremely configurable, Chrome wants to push out updates to its JS parser every month, there are like a million reasons this isnt done in hardware.
