It is not that dissimilar as branding in the real world. There are a ton of great little restaurants, but when you don't have time to really explore, some many small restaurants are also horrible, you pick a brand you are familiar with. The reason is that you know what to expect.

Similar to why so many people want to get Google, Meta, Microsoft, Apple, etc on their resume. While there are a ton of amazing small companies they just do not have the same brand recongition.

I think the economic term for this is "signalling." Having associating with these brand name schools gives you instant credibility: https://en.wikipedia.org/wiki/Signalling_(economics)