I think it has more to do with the gaming industry changing. Since so many people are starting to play video games, a company needs to design their games to cater to everyone, especially the more casual gamers which would encompass the majority (or at least a very significant portion) of their player base.
If the game was designed as you said, with ridiculously overpowered bosses, complicated systems, etc. The players who aren't as dedicated to the game would be no chance to ever experience that content, and knowing the nature of people, they would complain about it, and state that it isn't fair, which would lead to a lot of people quitting the game.
That's just how I see it at least.