Because it greatly simplifies the issue. It's basically saying "there would be more women in tech if they wanted to". And it completely ignores the issues of "Why don't they want to be part of these communities?" And if that question is asked, the answers usually circle around "Women shouldn't be offended by sexism." I can see how those two combined attitudes could offend people.

It's not about fixating on a 50/50 ratio. It's about changing the climate and attitudes so it's a more welcoming environment, so they do want to be part of the communities. If more women go into tech, that doesn't automatically mean less men. My personal belief is that it would increase the amount of capable people in the respective communities.

What I find most surprising though, is the vehemence with which these attitudes are defended.