Why is the gap concept a tournament theory?

[Felix Felicis]

Does it not take a better hand to call a raise than it does to open the pot in a cash game as well?

Perhaps it is especially true in tournaments, because of stack sizes and other stuff, but I don't understand why it is called a purely tournament theory.

P.S.
This is inspired by another discussion in this forum I just replied it where I said I would fold K9s in a cash game to a raise and it occured to me that I I would raise with it if folded to me in a lot of positions which made me think of the 'Gap Concept'.