One thing to remember is that alliance lounge access is one of the big unique selling propositions of the United Club. Only United Club + a subset of Air Canada Maple Leaf Club members have lounge access across the alliance. Today DL Sky Club members have access, to the best of my knowledge, a single reciprocal lounge at LHR when flying DL. AA Admirals Club members have a more generous program with most international outstations covered when flying AA, and Qantas + Alaska lounges when flying those airlines. Alaska is a different beast than the big three, but generally follows AA rules but more generous domestically and less so globally. The UA program is far more generous, and I think that this is a vestige of how a decision was made nearly a quarter century ago.

At one point (+/- 1997) I think Air New Zealand Koru Club and Ansett Australia Golden Wings Club members had similar alliance-wide lounge privileges. At the time airlines offering paid lounge programs (AN, AC, NZ, UA) accounted for about half of Star Alliance (vs LH, RG, SK, TG). Would not have been surprised if this was when the alliance-wide lounge access rules were adapted. Could see LH having been outvoted about allowing paid lounge members access to member owned lounges, and to this day insisting that the rules be followed to the word. Not sure exactly what the Star Alliance governance structure looks like, but perhaps founding member United has been able to veto changing this rule?

Going out on an even longer limb, it might well be that United decided that issuing plastic cards costs $x/year and only a single digit percentage of lounge members are denied access to LH lounges...so it was easier to just issue a token $50 travel credit to the smaller percentage of those people who complain?

Remember the Venn diagram is quite small: United Club members who do not have *G flying on *A who want to access an LH-operated lounge. I suspect that the only lounge in the world where this small group might constitute a critical mass of people would be at DTW which is in the US, but lacks a UA lounge, and has an LH lounge albeit with limited hours.