The price makes a huge difference when you have dozens of them operating which is trivial with a decent hydroponics and smarthome setup. I also have a dozen boards just sitting idle ready to be called up to replace a failed one or use for a new project because they’re so cheap.
Not to mention the Arduino core is supported officially by ESP32: https://github.com/espressif/arduino-esp32
Who actually uses Arduino in production? Everyone just uses modules (for ESP32) or rolls their own using the Arduino board as a reference. They’re so simple the NRE is definitely worth the unit cost if you’re already rolling a daughter board at 1k units.
Espressif has excellent good low-level documentation and a ton of code on Github[2]. I haven't built a system up from code, but they seem as open as any microcontroller supplier without having to buy into an NDA.
[1] https://hackaday.com/2021/02/08/hands-on-the-risc-v-esp32-c3...
At one time, long ago, it did - mainly the ATMega8 platform that was at the heart of the Wiring project, which ultimately begat the original Arduino (and Processing).
But the ecosystem expanded; first via different Atmel microcontrollers (many which became so-called "official" platforms), then eventually to other microcontroller platforms (and Microchip, maker of the venerable PIC microcontroller platform, eventually purchased Atmel).
The ESP8266 is a part of that ecosystem today, along with other so-called "ESP" controllers:
https://github.com/esp8266/Arduino
https://www.esp8266.com/wiki/doku.php?id=start-with-esp-12-a...
https://github.com/espressif/arduino-esp32
It wouldn't surprise me to find out there is a core available for the Raspberry Pi - or at least for the SOC on the board...
Hmm:
https://github.com/me-no-dev/RasPiArduino
I looked to see if I could find any kind of "real" core for the Raspberry Pi or Broadcom SOCs but I didn't find anything other than the above, which seems to be in active development. Honestly, though, you wouldn't really use a Raspberry Pi that way; more likely you would use the Raspberry Pi as a development platform for an Arduino ecosystem platform, that would likely be attached to the Pi via USB or some other method.
People, especially beginners, are often confused by what the name "Arduino" means; as I've noted, today it's an ecosystem composed of many different platforms, some "official", but many more that are not.
...and this of course doesn't get into the whole "real" vs "knockoff" vs "counterfeit" debate (which is a whole 'nother story).