If my country is hosting the World Cup and it failed to qualify for the competition, I will never go to the stadium to watch any game. The majority of people that go to watch these games are from the host country. Apart from the supporters club of most countries, the stadium will be empty without the citizens of the host country. For the competition to be entertaining and profitable, host nations should be given automatic qualification tickets. Don't also forget that these host nations will also spend millions of dollars on football facilities, infrastructures, personnel, and other essential services. We shouldn't assume that it is FIFA that finances all the projects, so they need to be compensated with automatic qualification.
This is why the hosts join the games. But it is not the main reason, I mean remember Qatar? They lost every game and was gone, did people not watch the other games? Of course not, there were a lot of people from Qatar that watched other games as well, some people will not watch them of course, but many will want to be there and watch it. Imagine world cup finals and you have a ticket, how could you not go? I mean doesn't matter which team you support, you gotta watch that
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This is the reason why it's both important to have the hosts involved at first, but doesn't mean that they have to win or go somewhere, it would need to be just in there, after that they can lose all their games and it would still have a lot of people who join and watch the games in stadiums anyway.