I don't think this is a system problem as much as a tactical problem from the DM. Use encounters that will put the ranged characters in harms way more often or in a tight area that will remove there ability to attack from far away, or have the enemies surround the players. If the player's use range to their advantage, so should some monsters.
Yes, you should not kill them all the time but you have to make it difficult for them. Just because the Fighter is a defender does not be you have to attack him. Enemies do not always come from the same side, reducing the effectiveness of the Fighter to tank all of the enemies at once. Not all monsters are dump, they understand that attacking the fighter would be easier (well, probably not, but they would not be hindered from the mark) but they also may know that dropping the strikers/controllers/leaders first is ideal to their survival. Otherwise, you combat is just like the Gauntlet video games were wave after wave of monster rushed whatever is closest to them.