End of the day, money is what changed stuff. Games suddenly had budgets like Hollywood blockbusters because big publishing houses were willing to toss more money at projects in the hopes it would equal big returns. With more money comes more expectations, so you need to bring on more people to meet those investor expectations. You also had the "Peter Principal" thing going on where your former MVP devs get promoted for the good work they did on those prior games, leading to new people coming in and those former MVP devs now primarily focusing on management and team coordination instead of actually making stuff themselves. Or those former MVP devs figure they will go it alone and leave (with mixed success).