I pay so that I don't have to add yet another thing to my increasingly limited free time. The older I get the happier I am to pay extra so I have to do less.
Because powder detergent (no matter if for laundry or dishes) has a nasty tendency to pull in moisture from the air, leading to it clumping up and getting ineffective. If you have a family with small children that produce a lot of dishes and laundry, you may get away with it but if you're single / DINK no way.
> In the same vein, antifreeze now comes "premixed" with water. Gaaaaah. I just keep looking till I find the lonely 100% antifreeze jug.
That one is because most people don't use distilled water to dilute, but tap water instead... which can carry serious issues, either because it dissolves the piping (see e.g. Flint water crisis) or because limescale builds up in the engine where it's hot and eventually clogs up, leading to engine failure.
The same for washing machine detergent. Why anyone would pay $$$ to have water come with their detergent is beyond me.
In the same vein, antifreeze now comes "premixed" with water. Gaaaaah. I just keep looking till I find the lonely 100% antifreeze jug.