Here’s my personal opinion.
A lot of us get into fantasy and sci-fi when we’re young. In my case, maybe well before I was ten. While there are many good authors, I do feel that many authors ride on the existing world-building and tropes to write their novels. The actual writing quality of many books is suspect.
When I read over the books I enjoyed as a child or young adult, I’m pretty surprised at how poorly some of those are written. I honestly didn’t care too much when I was young and I just inhaled books. As I get older, I’m much more critical of poor writing. I know enough now to criticise pacing, or development, or eloquence.
Finally, of course our personal taste changes over time. Many of us read these books at a super early stage of our development.