Why? I can answer that: look at the way men are berated in society:
We hold a door and get told off that "I can do it myself"
We compliment how a woman looks and get told off as creepy or worse sexual harassment
We get up to offer someone who appears pregnant a seat only to find out they aren't really
We get up to offer a pregnant woman a seat only to be told off yet again
Society can't have it both ways. Sure, wolf-whistling, cat calling, and genuine sexual harassment are all wrong and we should call them out, people being nice--like holding doors and offering seats--should be met equally politely.