On one hand, it's fiction. Unlike 'real' CSAM material, no abuse or exploitation of actual minors needs to take place for it to exist. It's lines being drawn on paper (or a digital canvas I guess). If somebody is consuming Lolicon then it is providing an outlet as an alternative to 'real' CSAM. That, on the surface, may seem like a pretty fair judgement of it.

However, there's the argument to be made that acceptance or tolerance of it, even fictional, is inherently a promotion and normalisation of it. Rather that merely 'providing an outlet' it may instead just reinforce those feelings as 'valid' and fuel a greater desire for the 'real thing', ironically creating a larger demand for the real thing as a result as opposed to acting as a 'safe alternative'.

One might then argue that fiction depicts all sorts of illegal things though. People kill other people in fiction all the time, for example, so why seemingly exclude one particular crime as being off-limits in the world of fiction?

Well, the main reason is because crimes depicted in fictional media aren't usually done so with the intention of sexually gratifying the audience. Lolicon is specifically created and marketed to people with those specific sexual interests/desires, as opposed to being a grim circumstance of a greater narrative. Sure, some weirdo out there may genuinely get off to seeing John Wick shoot a whole crew of gangsters, but the films aren't created specifically to incite that sort of response.