Why do authors do this?

There's this thing that I don't get in a lot of stories that I read. It's where they show you the mc, introduce to the characters, show you the mc's goal (usually revenge or becoming stronger) and what's stopping the mc from reaching it, then the mc goes through some really bad shit, gets over it and in the end finally reaches their goal in the end, and you'd think it would end there because by then there would be a lot of chapters and literally every villain has been dealt with, and every side characters had their fate sealed (got married, died wtv) but then somehow the story still continues?? They just keep introducing random variables and random enemies and random shit despite everything being already done with and it's just totally unnecessary, like the story ended already!! Let it be bro