Arbitrarily promoting the interests of those more similar to yourself is the basis of families, nations, religions and even ideologies, while all of these go wrong occasionally (especially religion and ideology are unlikely to have many friend here), I think that overall they are a net gain. People help those more similar to themselves practically all the time

Yes, this makes a lot of sense due to coordination costs; we are less informed about folks dissimilar to ourselves, so helping them effectively is harder. Reflexive consistency may also play a role: folks similar to myself are probably facing similar strategic situations and expressing similar algorithms, so a TDT decision rule will lead to me promoting their interests more. It is easier to avoid hurting others when such situations arise than to try and help everyone equally.