Why does scratch not have arduino when it should and can

I agree!

I do understand that maintaining scratch is a big task in itself, and from looking at the list of extensions they show messages like “In collaboration with Lego/micro:bit/joylabs”. So it seems like the Scratch team has just taken an approach of letting the creators of different hardware lead these types of integrations.

That being said, that doesn't stop ambitious people from trying to add a bridge themselves. You likely have found the old S4A project, but that is built against an old desktop version of scratch. I thought I might share the project I've been working on that involves connecting scratch to Arduino but in a slightly different way than that one. While S4A was focused on providing a way for Arduino programs to communicate with an Arduino, with the Scratch code still running on your computer, and requiring a connection between the computer and Arduino board, I have been working on a way to convert Scratch games into Arduino C++ code, so they can be run completely independent from your computer once you upload them to the board. The project is still in early stages but several scratch games I made as test cases convert pretty well (there are still bugs even in those games, so this isn't a “done” project that works for everything). Would be very interested to hear back if anyone tries it out on one of their games!

https://scratch.mit.edu/discuss/topic/840113/?page=1#post-8727228