Has my brain rotten from too much fast media etc., or is there something else going on.

Very probably, yes. Though not "rotten" so much as your attention span currently shorted. Books have a significantly slower investment curve than other media and do not provide that immediate endorphin drip that the novelty of social media provides nor the swift hook of music and movies. I realized that something similar happened to me several years ago when my eyes would stray before completing a single page. But fortunately, it is something you can likely reclaim relatively easily. Keep your phone in another room while you read, explore audiobooks (or read along with an audiobook), set a schedule, persist, etc. It also helps to read along with a club or a friend as a social component introduces another avenue of reward in your circuitry and keeps you committed.

Idle musing here, but we probably also tend to have better expectations about other forms of media than with literature. Movies and television and games have trailers or trusted studios, friends can have tastes that are familiar and known to us, and genres can perhaps tell us readily what sort of enjoyment may be found in each. Even though, say, a science fiction novel can have similar setting and themes, it might be a character study, a mystery, an anti-capitalist thesis, philosophy, etc, which is not as readily apparent while you are in early chapters. While (especially combined with trailers or promotional material,) we tend to know to be titillated by explosions in action flicks, humor in comedy, twitch reflex feedback loops in FPS's, etc, etc. We also speak about books on a longer timespan. I can tell you I liked reading Beowulf in high school but for all I know, I would hate it now. But you are more likely to be recommended music or television people have consumed recently and can speak more concurrently towards.