I read that Vilna Gaon emphasized the need to read the Shema from the Siddur rather than to say it by heart. This from a man who had a photographic memory of Gemara. I assume the reason he insisted on this is the chance that we could unintentionally leave out or misremember passages we have been saying since we were small.
Or it might be because the text of Shema isn’t in the Gemara.