There are instructions on how to install Arduino framework as ESP-IDF component: https://github.com/espressif/arduino-esp32/blob/master/docs/esp-idf_component.md
Why not make it installed by default?
Noooo keep it away from me!!!
me21, I think it would be better to develop and manage ESP-Arduino and ESP-IDF separately. Tow projects have each own mission, I believe. It looks ESP-Arduino pursuits Arduino compatibility using Arduino API but ESP-IDF does not. ESP-IDF is trying to do maximizing usage on FreeRTOS and use its benefits - several tasks are running at the same time.
Sorry for the late response on this! Including Arduino-esp32 into ESP-IDF as a component would complicate licensing and also add more test items to ESP-IDF release flow, so for now we don't think this is a workable option. We are however working on improvements to installation flow of such external components.
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Noooo keep it away from me!!!