Oh they are.
But heat pipes/vapor chambers have them beat.
So they are only used where electrical insulation (pure diamond sheet) or mechanical strength (cheap diamond grains sorted into a densely packed single layer of diamonds embedded into a copper matrix; this fits nicely for silver sinter bonding dies to due to the low thermal expansion of diamond) is needed from the heatsink.
Oh, nice; I was thinking of what I believe is this, though: https://sumitomoelectric.com/products/cu-diamond :
size-selected diamonds to match the thickness profile of the power module/brick base plate, then encased in copper.
Benefit is arguably more the expansion matching to GaN/SiC than the thermal conductivity improvements over plain copper.
