I believe it has been a tradition since the initial world cup to have the host qualify automatically.
I mean, can you imagine hosting an event in which you're not even participating?, doesn't seem too fair to be honest. The hosts need to put a lot of money and effort to make it happen, at a minimum they should qualify.
Your logic works when one or two countries are hosting the world cup. But it doesn't work when a large group of countries are co-hosting this tournament. And in 2030 it is going to be even worse. There is a combined bid from Uruguay–Argentina–Chile–Paraguay. If they succeed, then there will be 4 co-hosts and each one will qualify for the tournament automatically. And for all the other teams from CONMEBOL (including Brazil), only 2 spots will be made available for the 2030 World Cup. Do you really believe that this is fair?