Why shouldn't airlines charge excess? They make their money charging for carriage of passengers and baggage/freight. If you choose to bring more baggage than you've paid for (when purchasing your ticket), of course you should pay more - it's not just about cost covering max AB (do you think courier companies charge by weight simply based on what it costs them to carry it and how much extra fuel the'll use). As Led Zep said, baggage weight often directly affects the quantity of freight that can be carried.
I'm quite happy for people to pay excess baggage charges if it keeps the ticket price down for the vast majority of us who travel with a reasonable amount of baggage. Unless you're moving house, I fail to see how it's so difficult to stick to 20-25 kg.
As for charging based on pax size, this is probibly not going to happen due to human rights considerations. I do like the Southwest policy though.