Because most contemporary, 21st century feminism - like nazism - has decided to scapegoat men for all the problems in the world and has, like the nazis, generalised an entire group of people (in this case men) to aim their hatred at for their illogical arguments surrounding equality.
Let's be clear about one thing: women are not equal to men currently. But that doesn't mean women aren't benefitting from that position. Women have reached a comfortable niche which is almost, but not quite, the same as their male counterparts, meaning they can complain about wanting equality in some fields - equal pay, less women in certain jobs (mostly CEO positions/politics etc.) - yet don't want to be treated equally in others - a women hitting a man is considered acceptable but not vice versa, women get off with a staggering amount in court, and win almost 80% of custody battles.
The majority of the arguments modern feminists make drastically understates the violence women in 3rd world countries suffer from. But of course, modern feminism is more focused on banning 'Blurred Lines' from a university campus than actually giving a **** about women being stoned to death in Saudi Arabia.
Modern feminism has reduced whatever legitimacy its predecessors had to little more than a couple of over-priviliged university students trying to silence male voices at any chance they get. And that is one of the biggest problems of feminism now: it attempts to silence anyone or anything which challenges it by accusing its opponents of everything under the sun.
Lest you agree 100% with feminists you are automatically a rapist, hate and beat women, and despise every leading female figure there ever was. That is why feminism is compared to nazism because it has become an ugly, twisted movement that no longer seeks equality but superiority.