That's not really the essence of ROP because other attack techniques often also need to manipulate the stack. The key novel idea in ROP is to use data in unintended ways. This is based on the insight that the memory often contains short sequences of bytes (e.g. a .jpg image) that can be interpreted as machine instructions. For example an mp3 file might contain the sequence 99 19933, 16 which translates to

    increment register 16
    return

in the ambient machine language. Call that "dual use data". ROP searches the memory for sufficient "dual use data" and then builds an ac-hoc compiler with "dual use data" as target language. Then the attack software compiles to "dual use data" and then runs the compiled code.

Of course one may ask: can we always find enough "dual use data" to build a Turing-complete set of instructions as a compilation target. Turns out that with high probability that is the case.