You certainly have a point from where you’re coming from, but if you try to assess that from Fairphone’s point of view you should easily realize why they don’t do this.
The glass and the display are “fused” (quoting iFixit’s teardown). I want to assume there’s a reason for that other than milking money out of people. Dust perhaps?
So you as a company, by offering to your customers the glass alone and the glass alone probably being significantly cheaper than the fully assembled display unit, would encourage your customers to not very straightforwardly play around with hair dryers or heat guns and whatever tools with a sharp edge to get the glass off the unit, which may possibly fail in several ways, probably leading to a number of affected customers being pretty unhappy about the whole affair, asking you “If this wasn’t easy enough for me to do, why are you selling the glass part to me in the first place? Now I have to buy the whole unit anyway …”? And not only asking you this, but giving you bad press about it all over the internet …
I wouldn’t do that, but then again I’m not Fairphone and I’m just speculating 
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And “stocking only the glass” for Fairphone would mean guessing how many glass parts could eventually be requested by customers, buying this lot, stocking the parts … when they decided to bundle the whole display affair into a fully assembled unit which can be easily replaced by every customer. Probability not zero, but close to it, I fear.