Reality is more like:
Big co buys a big IP because they are excited to sell stuff to that fan base
Producers scurry around trying to find people to attach to the project
If they find a good director and/or team of excellent writers who are genuinely passionate about the source material, they are worried the artistic vision will frighten the exec producers and fat cats funding the movie.
If they find big name writers or director, that person frequently wants to make their mark and doesn't care much about the source material. If they can't find big name writers or director, the people they get are second-rate and can't help but butcher the source material whether they care about it or not. After all a film/series is very different from a book/book series and so some changes/adaptations have to be made to accommodate the new medium.
Along the way the project drifts while different stars and funding fatcats are attached to the project causing writes/rewrites, additions, changes etc. Later the project drifts when people see rushes or tehre are test showings etc and they decide rewrites or reshoots are needed.
It's very seldom that a talented director and/or team of writers is given a genuine chance to produce something true to the original source material. I mean if you think about Fran Walsh and Peter Jackson, prior to LOTR they were most known for writing and directing a pretty non mainstream horror movie. The sort of risk-taking that it takes to greenlight them doesn't happen very often.
Big Co buying 100's of Big name IP titles / options... BIg Co has no real plans for the IP
Big Co has lots of screenplays not attached to any Big Name IP
Big Co randomly pulls Screenplay Y, and tells screenwriter to make this "into a This big Name IP" and slaps the Big Name IP title on it...
So many of these movies and show have soo little to do with the orginal source that they have to been imaged as wholely new properties, but then retro fitted with the name slapped on to an existing property because Hollywood is completely adverse to any new thing, it all has to be a reboot, sequal, or prequal at least in title.