It’s not so much discrimination as law firm selectivity. Firms have a sliding scale of minimum gpas depending on the law school, with top firms requiring students have a higher gpa from lower ranked schools to get an interview. For lots of students interested in the biglaw paycheck for a few years and solid exit strategies, not being able to interview at Wachtell because they don’t come from a high enough ranked law school is not relevant, as long as they can get a job with a good enough firm. I think the Vault law firm rankings should be used the same as US News college rankings – look at tiers and don’t get caught up in whether Millbank is really a better firm than Covington, just like it would be silly to obsess about whether Bowdoin is really better than Carleton.
Also, while the top ranked law schools tend to have higher federal clerkship rates, clerking from outside the T6 as well as T14 happens regularly (though that leads to a whole other discussion about the role of FedSoc student membership as a pipeline to clerkships and how that pipeline makes hiring by non-FedSoc judges even more competitive).