It is for the same reason that `rg`/`ag`/`pt` are used nowadays instead of `grep`. 99.99% of the time, you don't want to search files that are ignored by your VCS, and you want to search recursively, and also probably in a case-insensitive way. For `grep` you have to fight with tons of options to filter out files, to enable recursivity and to ignore case. With `ag`? just `ag whatever`.
Defaults make the difference.