Then why surrender at all? The emperor does not want his people to die from atomic weapons but is fine if they starve to death?
Because what the atomic bombings represented was something worse; an extinction that Japan would be powerless to stop.
Japan had been bombed plenty of times before Hiroshima, and in some cases (like the firebombing of Tokyo), these raids killed more people than the atomic bombings. With that knowledge, it stands to ask; why was the atomic bombing such an insurmountable threat that Japan was forced to "bear the unbearable"?
Because, simply put, the atomic bomb represented a latch key game changer. Japan had been bombed relentlessly before, but during raids which required hundreds of bombers dropping tens of thousands of explosives. Compared to a single bomber dropping a single atom bomb, the atomic threat looked like a sudden escalation in the capabilities of the United States to harm the Japanese home islands.
We know now, of course, that atom bombs were in fact extremely costly to produce, and in fact were almost hand made given the complex and often theoretical design that went into them. It cost more to make, field, and employ single a atomic weapon than it did to construct, arm and conduct an air raid of a hundred B-29s.
But the Japanese didn't know this. All they knew was that the Americans had a bomb that could do what it took a thousand bombers to do for the same level of destruction. When preparing for the invasion of the Japanese homeland, the Imperial Japanese High Command was expecting to face waves upon waves of bombers that would blot out the sky, shielding a vast invasion force of thousands of ships carrying millions of troops. They had never expected instead to face a handful of bombers dropping a handful of bombs, but inflicting essentially the same catastrophic damage.
It's important to realize that an invasion of Japan as forecast in Operation Downfall would've fit in perfectly with the Imperial Japanese mindset. While they did not want an invasion of Japan per se, such a battle, a final clash between the land of the rising sun against the foreign devils and barbarians would've fit in perfectly with the ultra-nationalist bushido mindset that had been imprinted on Japanese culture since the end of the Russo-Japanese War. This was the final battle, where the 1000 year Empire would establish itself as the true supreme power. It didn't matter how many Japanese were killed, at least to the national leadership, because it was seen as a preferable alternative than surrender.
But the big part of that was the idea that it would turn out just like Operation Downfall; a conventional invasion of Japan. Atomic bombing changed all that.
There would be no great battle. There would be no final, climatic struggle. There would just be the atom bombs, just a handful, dropped on the major cities and centers of population, that would reduce Japan and the Empire to nothing but a shattered wasteland. It would be worse than a surrender, it would be a defeat which they would be powerless to reverse or stop.
"Moreover, the enemy now possesses a new and terrible weapon with the power to destroy many innocent lives and do incalculable damage. Should we continue to fight, not only would it result in an ultimate collapse and obliteration of the Japanese nation, but also it would lead to the total extinction of human civilization."
-Emperor Hirohito
Note: I do believe that eventually starvation would force Japan to surrender as food riots broke out across Japan, but that would not be a quick thing nor would it result in only minor casualties.