A counter argument which will be well understood as self-justifying if made by a boot rom specialist:

Every machine I make winds up looking a bit different. The new bus has different logic. The chip initialisation differs. Blobs become more interestingly hard to handle because associated pre boot initialisation dependency keeps rising and no amount of push back from me stops it.

If I make my boot ROM forth, I can reduce my marginal costs to writing forth code for most variant handling and occasional uplift of new primitives and constants into the forth for edge cases. My life gets simpler if I implement the wheel of life.

I would imagine after the 10th sub variant, one would wind up thinking like this.

Of course a rational alternative is to maintain a monrepo of all the variants and recompile all of them all the time to make all the boot ROMs far smaller. But making the generic anything ROM and changing only some forth would be attractive.

 Never owned this problem. I did work with two groups doing lsi-11 images for x.25 handling on yorkbox, and they definitely thought more like you than me on this: hand code it, code it well, they aren't general purpose devices when doing this kind of job. (I annoyed them a lot which tends to "probably they were right" in hindsight on my part)