It's that everyone in the main cast is their own flavor of a Mary Sue. And all the girls have no meaningful autonomy once they meet Kirito.

Kirito: The ultimate perfect game player, who never gives up and can instantly master the mechanics of anything, solve mysteries, and only comes close to defeat when someone else is cheating.

Asuna: The perfect girlfriend-wife. Such a brilliant and smart tactician (that we never see, only get told about), Kirito's (again almost never shown) near-peer in gaming ability. Described by others for her peerless beauty, grace, and delicacy in the real world. Despite the discussed and noticeable PTSD she has from SAO and the Fairy Queen shit, Kirito is easily able to get her back into gaming because he asked. Oh, and they also already have a perfect in-game AI daughter, who is a legitimate afterthought later on.

Shino: The perfect mistress girlfriend (remember this is Japan, where sleeping around is at least partially accepted). Again, a skilled, but nowhere near Kirito's level girl, who...doesn't really have much more in the way of personality than that. After her childhood trauma is overcome...there's NOTHING THERE besides devoted mistress vibes. And of course she's a catgirl in the other game. Does she have her own interests?

Sugu: The perfect devoted little-sister for the audience inserts to lust after, and oh, she's actually his cousin so it isn't incest by Japanese laws. Doesn't seem to have any interest in other men aside from pining after Kirito, knowing that it's hopeless because Asuna is SO PERFECT, so she will remain an eternally pure virgin with unrequited love for her onii-chan. Does she have a personality? She did. She used to do kendo. Then, "she begins playing Alfheim Online as a Sylph in an attempt to try to understand Kirito better." WTF. She doesn't lust after him anymore, but she endlessly tags along after him.

Alice: AI perfect childhood friend girlfriend...you get the deal by now.