World Cups have to alternate with continental competitions (Copa America, Asian Cup, European Cup, Africa Cup of Nations) which are on similar cycles. They could technically be held every two years (and current FIFA leadership is pushing towards that), but federations and clubs are resistant (because every summer tournament places even more stress on an already-long club season, and it would likely devalue other competitions).
FIFA pays federations on a result basis. It's then up to the individual federations to redistribute that money as you see fit. As you can imagine, a lot of that does not end up in the athletes' pockets... I believe clubs get some (low) compensation for injuries, but it's also common knowledge that players who play deep into summer will end up performing poorly the following season.
