Two reasons... (bold typeface added for those who don't care about my rambling, not to seem like I'm shouting)
The main reason...
I've mentioned before,
I don't like using build docs when I build pedals. I'm spoiled and lazy, I'd rather everything be spelled out right there on the PCB.
There are a large number of boards here that could be eliminated if I added a long list of modifications to
other boards. (Substitute these parts, add a jumper here, cut this trace, omit this part, etc etc etc) Think how many of the 'Muff inspired circuits
could be built from the same Muffin PCB if you didn't mind wiring up an offboard pot with some caps and a switch and (the list goes on).
The cost is no different, so to me it's just as easy to start with a PCB with the correct parts printed right there on the PCB. There is another upcoming release that is identical to one of the existing boards, but the component list is completely different... You absolutely could build it with the existing board (and that's exactly how I prototyped it), but since the cost is the same, why deal with the inconvenience of translating part values?
The other reason falls into a "marketing" situation...
If someone comes along who wants to build that circuit, but doesn't realize what it's based on, they'd have no way to know it was right there in front of their face. I suppose I could list every variant possible on the product page, but that'd just get messy as well.
The extra boards take up about 4" of bin space, it's no burden. Modifying the PCB layout took about as long as it would take to work out which values need to be substituted and that only has to be done once, now no one else has to worry about doing it.
With that said, I'm all for listing the Glue Factory modification in the LGSM build docs. It's not about "selling more boards" or anything like that... If you're going to order one or the other
anyway, why not get the one that makes the job just a little bit easier?