There are several reasons why SOCs (system-on-chip) don't integrate WiFi. First, WiFi modules require additional hardware and antennas to support them, which increases the cost and complexity of the SOC.
Second, integrating WiFi may lead to electromagnetic interference and increased power consumption, affecting the performance and battery life of the SOC.
In addition, SOCs may be designed to focus on specific functional or application areas rather than general-purpose wireless connectivity. Therefore, for flexibility and cost-effectiveness, adding WiFi as a standalone module to the SOC may be a better choice.
The mcu itself is the soc. The mcu generally does not integrate wifi, but expands through peripheral interfaces. The wifi chip itself can be an soc, so the wifi mcu is the application layer of the soc.