Hard for a user to get a feel for what a PageRank score means, especially when there are many different ways to run it (different parameters, etc). If I as a user could be guaranteed to see the same PR score on zapstore as I see on amethyst and damus and primal and everywhere else, maybe over time I could get a feel for what score is a good score. And that can happen if we give users full control over how and where their scores are calculated. Direct client-to-vendor integration is contrary to that goal. But NIP-85 is designed with that goal in mind: giving users, not clients, control over which WoT service provider to use.