I find sequels/series are generally worse books, just based on the nature of what sequels/series are: primarily that the sequel/series structure means the story is less self-contained and less satisfying. I think a new book also requires more ideas and time and polish to get finished by the author/bought by a publisher. There’s also just basic “regression to the mean”: most books are average; if the first book in a series is exceptional, statistically any other book that follows is more likely to be relatively worse.

I think it’s also what leads to a sequel effect with reviews- if you get a number of people reading the first book, your second book’s audience is essentially limited to the biggest fans of the first pool of readers. So you tend to get a second book that’s worse, but with more enthusiastic reviews.

All that being said, I’m not against the concept of sequels/series- they’re just fighting an uphill battle.