The issue here is not standing but the fact that some such laws (which exist in other countries like Canada or Australia) that ban the mere depiction in cartoons of sexual acts with children were struck down by the US Supreme Court as infringing on freedom of speech, which in the US gets some of its broadest interpretations worldwide. To wit, former LA prosecutor Neil Shouse writes:
Under federal law, animated child pornography may be an offense. Congress passed the “PROTECT Act” in 2003, which outlaws anything that contains “a visual depiction of any kind, including a drawing, cartoon, sculpture or painting“, that “depicts a minor engaging in sexually explicit conduct and is “obscene” or “depicts an image that is, or appears to be, of a minor engaging in…sexual intercourse…and lacks serious literary, artistic, political, or scientific value.”
The PROTECT Act passed following the striking down by the U.S. Supreme Court of a prior federal law that made simulated child pornography illegal. The Court in Ashcroft v. Free Speech Coalition, 535 U.S. 234 (2002) claimed the prior law was too broad and violated the First Amendment since it “prohibits speech that records no crime and creates no victims by its production.”
Note that the First Amendment does not offer protection for “obscene” speech. Miller v. California, 413 U.S. 15 (1973). Therefore, images depicting child sex acts that do not involve real children can violate the PROTECT Act if the material is considered “obscene.” In its decision affirming the constitutionality of the Act’s outlawing of obscene virtual child pornography, the Ninth Circuit stated that “the fact that this statute does not require that an actual minor exist… is immaterial because… it is an obscenity statute and not a child pornography statute.” U.S. v. Schales, 546 F.3d 965 (9th Cir. 2008).
So although you may not face federal charges of “child pornography” for the animated or virtual depictions of child sex acts, you may still face federal charges if the material is deemed “obscene”.
As Wikipedia details a bit on this matter, the 2003 Act essentially added two of the tree prongs of the Miller test (itself devised by the Supreme Court to define obscenity) to the 1996 CPPA provisions that by themselves were ruled too broad by the Supreme Court.
However prosecution for obscenity is itself notoriously difficult in the US, per NYT (2007):
Despite stirring anti-pornography speeches by both heads of the Justice Department during the Bush administration — John Ashcroft and, more recently, Alberto R. Gonzales — there have been fewer than two dozen federal obscenity prosecutions that did not also involve charges of child pornography.
While I could not find any stats on [obscene] "virtual child porn" prosecutions under the 2003 Act, some do seem to exist. There's is one 2013 announcement from the Justice Department.
[the defendant] pleaded guilty to possessing an obscene image of the sexual abuse of children. The pornographic cartoon, which depicted children engaging in sexual behavior, is categorized as obscene and therefore illegal. The original indictment, which charged [the defendant] with receiving child pornography, was dismissed today as part of the plea agreement. [...]
During the forensic examination of [the defendant’s] computer, a collection of electronic comics, entitled “incest comics,” were discovered on the computer. These comics contained multiple images of minors engaging in graphic sexual intercourse with adults and other minors. The depictions clearly lack any literary, artistic, political or scientific value.
Interestingly, perhaps, the FBI was not [mentioned as] involved in this investigation, but only Missouri state authorities, although the defendant was sentenced in federal court.
There is however one 2006 case, which was actually highlighted on FBI's website as the first conviction under the 2003 law:
Whorley—who had spent time in jail on previous federal child pornography charges—became the first person in the U.S. to be convicted under the 2003 law. On Friday (March 10), he was sentenced to 20 years in prison and fined $7,400. [...]
Our computer experts in Richmond and from the Child Exploitation and Obscenity Section of the U.S. Department of Justice extracted evidence from the computer, and a Japanese linguist at FBI headquarters translated the text of the cartoons to provide further proof of the cartoons’ content.
FBI Agent Gerald Kim, who led our investigation in Richmond, said the cartoons were extremely graphic. “There was no doubt about what was being depicted,” Kim said.
The Japanese anime was just part of the case against Whorley. Our cyber experts found digital photographs of child porn on the same [...] computer. [...] He was convicted of a total of 74 counts of obscenity and child pornography.
So it seems the FBI first chose to use the 2003 law in a combined obscenity + non-virtual child porn case, which--as the NYT noted--makes the majority of such prosecutions.
I don't know a clear/stated reason why the FBI [probably] prioritizes these combo cases, but it's easy to guess/speculate why they do that: in cases that involve real children adding some years/counts of conviction using the obscenity charges is fairly simple and the defense is probably not likely to try to challenge the cartoon/virtual obscenity charges on "artistic" grounds when the defendant is [also] found having non-virtual/real child porn.
I guess a problem is that there is a surprisingly large amount of complaints that the authorities apparently have to sort/prioritize:
In 2019 alone, the National Center for Missing and Exploited Children received 16.9 million reports related to suspected child sexual exploitation material online. [...]
"Notables" are images and videos that have been reviewed by law enforcement officials and determined to depict children under age 12. The material typically comes from the seized devices of suspects or reports from technology companies. That, police say, rules out some material that either isn't illegal in every jurisdiction or isn't a priority for prosecution.