Why was the island hopping campaign in the pacific so deadly to both sides?

Apanese - resulted in 3000 Japanese polits dying, short on aircrafts because of kamikaze's and 83000 japan people dying

allies-50,000 casualties (A casualty in military usage is a person in military service, not necessarily a combatant, who becomes unavailable for duty due to death, injury, illness, capture, etc.; or a civilian casualty.)