Why is it impossible for anyone to package powdered miso soup for sale in the U.S.?

I can't believe that every Japanese restaurant I've ever visited makes their own miso from scratch, yet the worst miso soup I've ever had in a restaurant is better than the best I've ever bought in a mix.

What's the secret? Is retailly-packaged miso soup just a ploy to get more people to go to higher-profit-margin restaurants? Do they save the good stuff for Japan? Is it some obscure form of self-flagellation?

If anyone knows the secret, or if you know of a supermarket miso that breaks the rule (or a restaurant that breaks it, the other way), let me know, and I'll share what I learn.

Thanks!