IMO its because the Hololive girls exist in a weird sort of limbo. On the one hand, they're anime girls. On the other hand, they're avatars for real people. It would be like seeing drawn porn of some random twitch streamer.

The fact that many of girls look up their own lewds just complicates things any further (or in Haachama's case, reviewing them on stream). Or you have Coco who conflated the word "waifu" with fap material and caused all the fans to panic. Yeah, its a really strange dynamic.

There's just some sort of effect that Hololive has that makes it hard to feel horny for them after tuning into their streams.

Like sure I'd fap to hundreds of Miko Lewds and feel so great, but the next day I'm laughing at her dying to lava and accidentally turning on her ero-games. There's like a cut off from the horny mind when it comes to watching their streams and clips after a whole day of viewing horny art.