Having more countries will cut the costs of building new stadiums or improving the actual ones by only one nation to having multiple nations that will divide this charge while enjoying more profits from having more games.
It is all commercial business.
First of all, a major tournament such as World Cup should be only hosted by countries who have the means to do so. I don't have any issues with countries such as USA, France, England or Japan hosting this tournament, because they already have the facilities available. On the other hand, if you allow small countries such as Qatar (total population of just 300,000 citizens) to host the tournament, then a lot of infrastructure needs to be created newly. I would consider this as wasteful spending. Because after the world cup, most of these new facilities will be kept idle.