I think it's important to understand the early development.

It's true that you can (and always could) use avr-gcc and libc, but the core sale was what makes it not this.

The "locked in"/captured API and IDE were directly extensions of a language and IDE called Processing.

Processing overlaid an art-focussed layer on top of Java, providing a simpler API, and an IDE with just two buttons.

Arduino was based on this - the same IDE format, similar API conventions (just on top of C++), precisely to allow these same artists to move into physical installations and art.

Arduino was not designed initially to be so general, it was tool written by and for this specific group of people, so has opinions and handrails that limit the space to provide the same affordances as Processing specifically.