The problem with text is that it is hard for the brain to convey its meaning and work out the text image at a time.

Depending on your learning universe (visual, auditory or kinaesthetic), your mind will memorise the text you read in one way. Visual thinkers will remember the letters, auditory thinkers will remember the sound, and kinaesthetic thinkers will remember the meaning conveyed. Therefore, it may be really dificult to keep the text image still in your head, especially without external input. That will make the text blur and change, and recalling what you've read before may prove even harder than reading it again.

I wouldn't say it is impossible to read the same sentence repetitively, but for sure it takes practice.