They do get better as the series goes on but you also have to take the time period that they came out into context.

Middle and Young Adult books were very, very different back then and were a pretty small market. The success of Harry Potter changed that and is the reason why you're seeing so many books in that genre being published and pushed.

I agree that if Harry Potter came out today then you wouldn't get the same reaction.

A lot of the success also came from the very, very active fandom. There was fic, artists, musicals, bands, etc. A lot of people got into the books for the fandom. Hell, the fic alone made it worth reading because there are some God Tier fics out there for any pairing you could think of.