Why did the ancient Egyptians build pyramids

Mostly for security. Pyramids were obvious targets that got robbed no matter what, so New Kingdom pharaohs switched to hidden rock-cut tombs in the Valley of the Kings. Changing religious ideas about an underground journey through the afterlife, and the cramped landscape at Thebes, pushed the shift too.

No. The builders were skilled paid workers plus farmers doing seasonal labor when the Nile flooded their fields, not enslaved people. Archaeologists found their villages, bakeries, and graves right beside the pyramids. The slave idea traces back to the Greek writer Herodotus and later readings of the Bible.

Mostly for what the shape meant. The sloping sides stood for the mound of creation and the rays of the sun, giving the dead pharaoh a ramp to climb toward the sky. The wide base and tapering form also kept the huge stone structure stable enough to last for thousands of years.