Same reason they screwed other stuff up. Some stuff that's wrong with theirs or others' FMs could be (is probably) simply because of consumer hardware constraints. I think differently inasmuch as I think both are plausible till some way is found to figure out which is right. I'm just curious if you know that an absent vstab can e.g. mean it's past a certain surface area threshold, or if it really does mean the vstab area is truly gone but for those little beams.
Simplest way for you to see what I mean is to try for yourself. The ones I experienced myself were C205 and 152H. I didn't get to test the 152H, but did stay in flight with the 205 for a while and could do some rudimentary maneuvering. It appeared to have enough stability to pull off some useful maneuvering, with enough practice. And then there's the P-63 occurence which I described.