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There are some reasonable reasons, like how it may speed up passing regulations. I believe RPi used some integrated WiFi modules that had certification done separately. And yeah, there's a lot of passives in those modules that I would not want to manually assemble

The search term you want is modular certification.

The volumes necessary to justify the cost, time, risk, and specialized skill set necessary to achieve both regulatory and standards certification of a WiFi device are significant.

If you're referring to the module on those boards, I can tell you it's because the actual IC was impossible to source. Only modules were readily available.

Cost. You can easily get the premade ESP8266 boards for a quarter at scale. Laying out your own design for an ESP32 costs a lot more in time that's unnecessary usually.