Because they comprise three or four weeks of incredibly tedious monotony and if they were held any more frequently undertakers would not be able to cope with those that had died or boredom or committed suicide from the tedium.
The World Cup: Two years of qualification competitions. Then 64 matches to get a winner from 32 teams (more than twice as many as necessary) over four and a half weeks. What a joke!
The Olympics: Three weeks of tedium. Once you've got to the seventh heat of the Men's 200m highboard butterfly or the sixth heat of the women's underwater freestyle ironing you head for the gun room. Then, just when you think you've got through it, the bloody Paralympics kick off!
Interwoven with all this are European Championships, the Commonwealth Games, various World Championships. You couldn't possibly hold these events more frequently; the death rate would cause uproar.