A lot make rack mount ears for the smaller amps and those still follow the standard 19” dimension.
The standard 19" WIDE dimension, which makes them technically rackable.
I'm not going to look up every Class D head available but take the Mesa stuff for example; the larger heads "WD?" are FOUR inches tall; 2 rack spaces is 3.5 inches tall, so you have to lose another space above the head if you use the rackmount kit.
Reality is most of the people buying these rackmount kits are putting these heads into existing racks they already own.
Now that Class D rules the marketplace there is simply not a market really for making a cabinet with a "rack" space for a class D head. They are all different sizes. there is no "standard".
There exist half rack, third rack and also vertical rack formats (mainly for small studio outboard stuff) but none of these formats fit Class D heads.
The assumption of the companies was you would just put these on top of your cabinet, or if you really insisted they will sell you optional rack ears to manage to fit the thing into a rack.
It would have been simple to develop single space, Class D rackmount heads but that never happened because at the same time cabinets got smaller and away from the typical 24" wide format.
But sure there have existed various ways to rackmount non-rackmount stuff for years, rack shelves, velcro, etc. but there's simply not enough of a market for a cab maker to design a space to accommodate this stuff that is not a regular rack format.
IF these guys had all agreed on a head size format (say one size for smaller stuff, double height for larger stuff like the Mesas) then it would be easy and maybe such a product would exist, but they didn't so the product does not exist.
Analogeezer