Because it is an agreement that when the book is given to a human the shinigami must kill its user when the time comes. And because Kira was smart but not smart enough to outwit Near.

In a way i think he deserved to die because killing his own father and a few other innocent people.

But i do believe he was the true god of the world in the show. He stopped wars. Crime went down 80% and the world became almost peaceful

Age 16-17: Get job at Blockbuster

Age 18: Graduate, Degree in Business, and computer tec, Do student plan with Georgia National Guard, (Its where class comes first and i do basic training on weekends i choose when)

Age 18- 19: join National Guard fully (off student plan)

Age 20ish: Graduate from North West Tec Military Collage(One of the highest rated in the country) Using my collage fund as i need( National Guard pays for books, shelter, food, Plus i get around 300-500 a week to support my self,And this is WITHOUT A JOB)
Get AIT in computer Software and become Battlefield intelligence officer (being in ROTC i start as SGT or if i go to Officer Collage i start as Warrent officer)