I think runescape lacks these huge bosses because it is not as profession/team oriented as other games. Other games have professions specifically designed to take damage, or do damage, or heal others. In runescape, people are pratically all the same once they get high levels. This means you cant have those balanced teams to take down huge bosses. Its also becuase healing in runescape is pretty limited. In other games you get skills that heal for a lot or heal over time, with unlimited uses, instead of just a max of 28 slots for food/potions. If jagex got more involved with using magic to boost stats or heal yourslef or others, it would be more realistic to have these huge bosses in the game.