Why don’t people remember their childhood memories (one and a half years from birth)?

Not remembering childhood memories is called infantile amnesia or childhood amnesia.
The hippocampus and the cortex of the brain are not well formed until a certain time, slowly it starts to mature. They mainly work to retain and recall memories in the brain.
At that time, the child’s brain mainly works to build semantic information or important memories such as learning the mother tongue, learning different expressions, etc.
And it doesn’t store as much episodic information or surrounding events.
Over time it gets overwritten by other memories.

Humans can also store and recall episodic information better when the brain is structured.

When something very important happens to someone, such as an accident, sexual abuse, or some other important event, it can become embedded or stored in the brain.

“Childhood amnesia – Wikipedia” https://en.m.wikipedia.org/wiki/Childhood_amnesia#:~:text=Childhood%20amnesia%2C%20also%20called%20infantile,given%20the%20passage%20of%20time.