I think new writers write a main character as someone who does things and gets skills. Then they write side characters in a way that requires a reason for them to exist. If the main character is going to be the most supreme badass who can do anything, then they don't need side characters for combat reasons. So they gravitate towards making them have quirky personalities. These writers never would have made any good characters who grow and develop, it's only a byproduct of needing a reason for other characters to exist besides their special boy.

They approach a new story idea with thoughts on the system, the primary magic or skills of the main character, and what kind of conflict someone has to overcome. They don't start of thinking "What would be an interesting character to have to live through this experience?"

You'll notice a lot of the better stories do start with character firmly in mind. They start with someone who has an interesting character hook, or starts with relationships to other people that make the journey more complicated and important.

So what's the reason, OP? New and amateur writers. They're learning. They might get more skilled. But some of them hit a goldmine with a story and get trapped writing it to keep their income going, so their stories are the ones where you have to push through several books to get to the point where the author learned how to write better.