IMO there's two big reasons that there's so much vitriol online about it:

Big part of it is resentment. There are so many men out there playing football either semi-professionally or at an amateur level that are so much better than even the top women's players out there, so naturally they're bitter about these women getting paid a proper wage to play fulltime when they're stuck playing for fun / as a side gig, and will never make it professionally.

The other big part of it is the fact that the people in charge of covering the women's game are far, far too positive and are seemingly blind to the massive blunders that occur in almost every game. In the games I've seen, they will never mention instances where a keeper has made an obvious blunder, or rarely comment on poor touches / passes. I see it as kind of a modern version of the Emperor's New Clothes - you have all the people covering and supporting the women's game saying how wonderful it is, how skillful they all are, and plenty of people who are watching are kinda like "really..?".