Came about when players figured out they could make more money marketing themselves than the team marketing them. Athletes opportunities to earn a living are very finite. Compared to the general population, athletes make about the same on average over the course of their very short careers that you or I make in ours, just in a much shorter time frame. So maximizing their ability to earn more income can mean a lot to a player with a short window.

Are there some drawbacks to that? Yes, of course. But you can't fault people for making a play to earn a living while their bodies allow them to do so.

Once players became their own "brand" then coverage of the teams became more about the brand of players on the screen than the teams watching them. Other factors played into this at the same time, like a more global economy and easier access to players/teams not local to where you are meant that people from all over the world followed the game with indiscriminate biases on who they rooted for.

It's easier to market James Harden to inner city Brooklyn than marketing the Houston Rockets. Brooklyn doesn't have any relationship with Houston, but inner city kids might really like James Harden, or KD or Steph etc