I would think it's simply a matter of practicality. Sure, they could use indoor stadiums during the winter.. but not every venue would have that feature.
Soccer, football, etc.. are generally played on a grassy field, outside. That necessitates the need for cooperative weather.
Would I say that the decision to hold these events during the summer is made with the specific intent of elevating emotions and provoking aggression in sports fans? No. It is simply the practical thing to do.
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