Question

Why did Siddhartha's father do to keep his son at home?

Answer 1

The father didn't. He wanted him to inherit his place and even expected to build a larger kingdom.

Buddha left because once he thought the perfect world was not perfect.

His father hid old age, sickness and death from his sight and filled his life with laughter and happiness, youth and wealth especially because of the prophecy that his son might abandon the kingdom. (It was the time before if you believe in God, you will be given eternal life and if not, you will be put in hell, you will be resurrected one day religion was not spread yet. Even though, he was surrounded by water God, tree God, people offering many things to many gods according to their belief, he didn't buy any of those either.)

The crown prince Siddhartha found the tip of the iceberg which was old age, sickness and death and wondering why this happened, how one could overcome those etc.

Even he stayed, the death would overcome one day and separated the beloved things and people. Therefore, to seek something before his death, he left the palace and family.

Because of his decision, he was able to save not only himself and his family but also many people, celestials and Brahman.

Ok…. you don't need to believe in celestial and Brahman parts.