While that is true, both Espressif and the Pico have their own SDKs, and they're really well written too.

The Arduino SDK is the simplest to use, sure, but the Pico framework (I don't have experience with the Espressif one) is extremely good, and the Pico's PIO is a godsend. I used it to implement 3 wire SPI (data bidirectional on the same wire) at almost 'real-time', which is to say, at half the speed of the hardware SPI controller (half the speed because the interface clock is put up one cycle and down the next; this also gives enough time for data shuffling).

Why does the Arduino SDK necessitate a huge markup on Arduino boards, when $0 of every computer I buy to run Linux on goes to GCC?

Its relevant, however, that the Bluepill and ESP8266 cores for Arduino were originally independent reimplementations by third party hobbyists, not made by Arduino. And Espressif themselves have always developed the ESP32 Arduino library implementation. They weren't completely freeloading off Arduino's work, and Arduino (the company and the ecosystem) heavily benefit from contributors of all sorts. Particularly in the case of Arduino and Espressif, they have been successful together.