Flashing can be easy, sure. Compiling that binary, including library management, is not, unless you’re using something like micropython. CMake is not hobbyist/student-friendly as an introductory system. (Arduino isn’t either, but platformio with Arduino framework IS! RPi refuses to support platformio sadly)

Arduino took over for 3 reasons: a thoughtful and relatively low cost (at the time) development board that included easy one-click flashing, a dead-simple cross-platform packaging of the avr-gcc toolchain, and a simple HAL that enabled libraries to flourish.

Only the first item, and a bit of the second), is really outdated at this point (with clones and ESP32 taking over the predominant hardware) but the framework is still extremely prominent and active even if many don’t realize it. ESPHome for example still will generally use the Arduino HAL/Framework enabling a wide library ecosystem, even though it’s using platformio under the hood for the toolchain.

Even folks who “don’t use Arduino any more” and use platformio instead are often still leveraging the HAL for library support, myself included. Advanced users might be using raw esp-idf but the esp-idf HAL has had a number of breaking API changes over the years that make library support more annoying unless you truly need advanced features or more performance.