I think the reason behind a dedicated slots for Supervisors of 4500/6500 chasis is due to the architectural constraints.
For example the Sup 720 in 6500 has 18 fabric channel (18 connections to the fabric), the line cards have only 1/2 fabric channels depending on their version. So if you have to make a supervisor independent of the slot you need to have 18 backplane fabric channels in each slot of the chassis for the SUP to work in all slots as compared to 1/2 backplane fabric channels per slot as we have right now for the line cards. Almost same should go for the 4500. And this might come across as an architectural constraint. This is the best explanation i can think of.