Having a separate string for each level of progress also lets you do other kinds of customizations: you could have a rainbow progress bar, or put little bits of encouraging text to the right of the progress bar, like "Almost there!" at 90%.
Essentially, you're making one type of customization (i.e., changing the symbols) slightly easier, at the expense of making other types of customization harder.
Having a separate string for each level of progress also lets you do other kinds of customizations: you could have a rainbow progress bar, or put little bits of encouraging text to the right of the progress bar, like "Almost there!" at 90%.
Essentially, you're making one type of customization (i.e., changing the symbols) slightly easier, at the expense of making other types of customization harder.