The idea of music is to show the notes, how you choose to play them is your job.
Use one finger, or your thumb, use two or three fingers, use a pick etc, is all down to the player to decide what works for them and the music.
Standard notation only shows the notes, where as tab shows the note and a position to play it on relating to the fretboard...so they show you one option.
That is one of tabs flaws, it shows the writers preferred method not yours. The main reason TAB is even in the book is because the publishers want it. They know that the book will appeal to a wider range if it has tab, rather than only just SN. So in a way they popularise the use of TAB and add to its attractions, thus making it use more widespread, so almost making it a marketing ploy rather than an educational one.
You are expected to get to a standard where such things as fingerings do not matter, the art of learning to play your instrument to a high standard sorts out the best approach to use.