Why the obsession for catch weight fights and i don't mean Pacquiao?

I read an article recently of a dream matches pitting Bernard Hopkins vs. Sergio Martinez, Brandon Rios vs. Victor Ortiz, Julio Cesar Chavez vs. Saul Alvarez and it's all at catch weight! Except for the Hopkins-Martinez matchup, I think the catch weight for Rios vs. Ortiz and Chavez vs. Alvarez are not necessary. Rios always gets to his lightweight fights a full welterweight. Ortiz is just a newcomer at 147 lbs. Chavez does not hold any size advantage over Canelo. Why can't these fights be arranged at a stipulated regular weight limit? What's the use of having these many regular weight classes if we are tinkering with them or ignoring them? In the old days, the smaller fighters just climb the weight classes of their bigger opponents and beat them. No quarters asked, no quarters given.