There are a bunch of reasons but the primary reason is that good ADCs are made using a different mixed signal process than microcontrollers. MCU ADCs are capacitive charge-balancing successive-approximation type which limits their sensitivity and precision.
Standalone ADCs also eliminate significant sources of noise like temperature fluctuations and electronic noise (the digital logic on the chip often runs at less than 1Mhz for example)
Not sure I'd call the silabs chips hyper exotic. The SIWG917 is intended as a direct competitor to ESP32s. It's a bit more expensive, but not unexpectedly so.
I'm not sure about WiFi, but JieLi (JL) definitely has a huge marketshare for single-chip BLE/BT. They are the origin of the infamous "the Bluetooth device is ready to pair" stock prompt voice.
There are a bunch of reasons but the primary reason is that good ADCs are made using a different mixed signal process than microcontrollers. MCU ADCs are capacitive charge-balancing successive-approximation type which limits their sensitivity and precision.
Standalone ADCs also eliminate significant sources of noise like temperature fluctuations and electronic noise (the digital logic on the chip often runs at less than 1Mhz for example)