Arduino is also orders of magnitude lower friction than just about any other embedded platform. Baremetal AVR? Set up your makefile, ICSP programmer, and find the right fuse flags or risk bricking your chip with the wrong clock settings. STM32? Set up the whole STM32CubeMx project and export it as a Makefile and remember to only add code in the correct areas? Baremetal ESP32/ESP8266? Doesn't exist, but you can use a FreeRTOS fork with special binary blobs. PIC microcontrollers? Set up Microchip's HUGE IDE and special compilers and programming tools.
A board supported by Arduino is vastly easier to use, especially for someone who just wants their project to _work_, not to learn the deep arcana of baremetal systems.
I don't know how many people have a high voltage programmer, especially the people that would choose an Arduino. I'm personally lucky to have found an AVRISP mkII that was being thrown away when I was a broke college student.
Either way, I'm saying that Arduino is _the_ microcontroller platform with the fewest gotchas for a beginner.
A board supported by Arduino is vastly easier to use, especially for someone who just wants their project to _work_, not to learn the deep arcana of baremetal systems.