I haven't seen it be the default, but vi is part of POSIX so it's omnipresent.
And that because vi was present and popular on commercial Unix.
Isn't Pico the usual default?
I have seen nano to be the default for most debian and fedora derived distributions from early 2000s. Not sure if things changed in late teens. Or maybe I am misremembering things.
I always confuse pico and nano, tbh.
on more consumer grade distros I've had to `export EDITOR=vi` before to get vi to come up for things like `crontab -e` and for `git` messages. But it is the default on most systems I've come across personally.