Mostly

The arguments issued by lawmakers introducing these laws, or courts upholding precedents, has been generally under two broad premises:

1) That virtual kiddie pronz is a step in a slippery slope leading to real action kiddie pronz. Related, that future technology will eventually make them indistinguishable, so both have to be banned or one day real action kiddie pronz will escape charges.

2) That fictional characters are persons too for the purposes of the law because they somehow represent real children; so their depictions contribute to the sexual objectification of real children. (ie, the Feminist argument regarding real action Wimminz Pronz, only applied to Kiddie Pronz)

Both arguments could be summarized by what you say, but both have different legal theories behind.

And indeed, cartoon and written Kiddie Pronz remain legal in the USA only because the American standard for speech is higher; and cartoon and written Kiddie Pronz fail the test that a real minor must be used during the production. But - don't believe that there aren't people trying to close this loophole!