I have fallout and fallout 2 in my library and installed. But the store still shows them as not owned available to buy.

Maybe you have old versions from before Bethesda acquisition. The have "classic" after title.

A problem that is fixable for GOG but it is too low in priority. I doubt they will ever bother to fix things and take the next step up.

Fallout 1, 2 and Tactics used to be sold here by Interplay when they still had the rights to the old games (and for making that MMO that never materialized). After these rights went to Bethsoft these version were take form the storefront (but as usual not the library of people who bought them) and were renamed to "Classic".
After a while Bethesda started selling stuff on GOG, including the old Fallout games, but they got new store IDs for that (no idea if that was a GOG or Beth/Zenimax decision), and did not give these to the customers of the old version (which was very likely a Beth/Zenimax decision). I also have only the Interplay version here, but they are updated from time to time, so I don't think there would be any benefit from rebuying the Bethesda versions (except seeing them as owned in the store and raising their ROI on the Fallout deal slightly).

The only 'benefit' I can see (as I also own the 'classic' versions) is that The Elder Scrolls 1 & 2 (Arena and Daggerfall) are included with the purchase of Bethseda's versions of Fallout.

But you also get them when you buy any other Bethesda game on GOG, so this benefit is somewhat nullified. :)

Yeah I also got these for buying other games (i.e. Battlespire and Redguard which aren't on Steam). But it is potentially a benefit, that's right.