If initiative let all your characters act first when it was high enough then it would be a mandatory skill, getting all your characters to act before any enemies get a chance to move is really powerful. Especially when you consider things like being able to lay down massive fields of fire/ice/lightning to control where enemies can move or that that summons automatically act after the people who summon them. Then you'd see an arms race of NPCs getting crazy high initiative just to stay competitive. Players would be assumed to be pumping initiative so if you don't then you are building sub-optimally, restricting build freedom.

I think the current system is a compromise of mechanics to allow combat to flow in a more consistent way. You can pump initiative to get one character to act first and get things set up for the rest of the battle as well as set the turn order of your pals but you can't trivialise encounters right off the bat.

A necessary loss of freedom in one aspect of the game to allow the rest to be more enjoyable.