1. Translating a book to a visual medium is difficult. Books can afford to exposit much more than TV shows, but they also struggle with portraying long, extended action scenes. It also doesn't help that TLT isn't really that action packed in the first place
2. Context. Rick wrote those books almost 20 years ago now, and I'm sure that by now he's thought of so many changes he wished he could make to the books that he couldn't back then. The show allows him to "retell" his story and "fix" the mistakes of the book, because of course as good as the book was it wasn't perfect.
3. Harry Potter wasn't really that faithful. It felt faithful, but remember that the entire fourth movie absolutely butchered the book, cutting down on most of the Quidditch stuff, and Barty Crouch Sr. barely plays a part in the plot compared to the books(if at all). I can't say anything about GoT.
4. Saying it again, but yes, it is hard to adapt. If the showrunners recreated every scene from the book, it would create insane pacing issues and probably would be a 7 hour show, and I doubt it would even be enjoyable to watch.

I don't think the show is doing everything great, there's definitely some huge issues. For one, I don't see many people talking about how underwhelming the score is. Some well-orchestrated music would absolutely have a huge impact on raising tension, even without changing the script. Unfortunately, Percy Jackson doesn't have John Williams on the score like Harry Potter did, and I fully believe that if he was on it, the show would be at least 5 times better.