As I was first writing this, I realised that I was unsure as to whether (in my own mind) exactly how many distinct item types there were (it could be anything from four to eight, by the most widely-varying interpretations I ran through). That's "Items" as opposed to other 'thing types'[1]. So I started this post again. Hopefully I've made it more readable without so many striekthroughs.
However, concentrating on the main are of contention, I suspect that the very first of the object types is what might be termed "Construction Materials". This includes raw stone, stone blocks, raw logs, wood blocks, metal bars and a couple of other materials. There's a reaction to get stone to stone blocks, wood to wood blocks, ore-type stones to metal bars, all internal to this category. There are also reactions to get stones, wood, etc into one of the other categories of Item (e.g. Trinkets and Wearables: that being one of the "may be one, may be two categories" questions I couldn't definitely answer but not relevant to this discussion).
The way I see it, Stone Block -> Carved Stone Block needs to be internally set up to be a Constructable -> Constructable reaction, much as Raw Stone -> Stone Block is. But the code defining this transition might be a bit obtuse to easily convert, especially as it was a very early implementation (way before I was playing DF) and possibly a little too embedded in archaic side-code.
Alternatively, it's not just one "Constructable" category, but a "Raw Constructable" and a "Processed Constructable", stones and logs in the first category and the various blocks and bars in the second. And there's no means to convert PC -> PC. This might be supported by the windmills needing logs, can't use wood blocks, and pumps need blocks, can't use raw-types. Although that could as easily be encoded in without any basic RC/PC division, much as you can't build windmills with stone vanes (or indeed any stone at all, even what might be considered the 'supporting' piece).
My take, probably is wrong in various key ways, certainly isn't something that Toady doesn't know (even if correct), but might be a handle on why it's not a trivial change.
[1] e.g. "Agents" (beings, creatures and vermin instances), "Natural Landblocks", "Fauna" (pre-harvesting), "Free liquids" (liquids in containers are Items after their own 'harvesting'), etc.