What is the purpose of having a motherboard instead of one large circuit board?

The answer is about flexibility, repairability, upgradeability, and manufacturing practicality.

If every component were permanently soldered onto one board:

• Upgradeability: want a faster processor? You throw away perfectly good RAM, storage, and everything else. Modular sockets let you replace only what you need.
• Repairability: if RAM fails, replace the RAM stick. One failed component on a single board could mean replacing the entire device.
• Manufacturing variety: different users need different configurations. Standardised slots (PCIe, RAM slots, storage connectors) let manufacturers combine components in countless ways without redesigning the entire board.
• Cost: designing a unique board for every possible combination of components would be economically unsustainable. Standard interfaces let component makers build to a common standard.
• Thermal management: spreading components with sockets allows better airflow design.

Note: Apple's M-series Macs use a System on a Chip (SoC) CPU, RAM, and other components closely integrated. This sacrifices upgradeability for speed and efficiency a valid tradeoff for consumer devices with fixed lifespans, less so for enterprise servers maintained for years.