[sesqu]

people valuate injustices that happen to them much higher than things that happen to other people

That doesn't matter. Sure, people do disagree on things that involve them personally, but they aren't the only ones acting. Very often, people make judgements about matters that may later involve them or people they empathize with, on either side. These future people may or may not have had previous experience with the thing, but all that is required for them to act so as to lower expected injustice is that they empathize with someone on both sides. This act can be both unjust and reduce injustice, as long as the process is ongoing (so that the expected injustices and participants materialize).