One problem is that not all airports have showers. If you've had a rough day then what are you supposed to do? Sometimes you may have spent all day getting to the airport and you simply may not have had access to a shower. I don't think it would be right of the airline to deny people boarding unless some sort of shower facilities can be provided at the airport.

Sometimes I know that I will go hiking during the last day of a vacation before heading to the airport. I then specifically look for accommodation that will allow me to freshen up after check out, unless I know that I will be able to do so at the airport.

I certainly don't expect everyone to plan ahead that much. I just accept the fact that I am using public transport where I might have to deal with crying babies and smelly people.