No one has "invented" that it's untestable. String theory literally doesn't give predictions that would allow it to be falsified. There are an astounding number of possible geometries that mean that any set of data could technically be "consistent with string theory". There are no specific energy bounds where it kicks in - if an experiement doesn't show existence of strings, it's "try again with more energy" instead of "falsified".
String theory can't be disproved even in theory. Spending decades on that is akin to spending decades rearchitecting your software to make it more beautiful and full of design patterns and promising users/management that some day it'll all make sense.
String theory can't be disproved even in theory. Spending decades on that is akin to spending decades rearchitecting your software to make it more beautiful and full of design patterns and promising users/management that some day it'll all make sense.