Because you hardly ever want to grep through your build artifacts or node modules? I don’t remember any one time I had to explicitly tell ripgrep to search through all the files in a repository
Even in an insane setup you generally don't want to search both at once. I find the pattern of first grepping non-ignored files and then cd-ing into my build directory or whatever and rerunning more helpful anyway.
I will agree that in a sane project setup you don't need to search through all files including build artifacts ignored by git.