Why does it seem so many hate people who choose not to have children?

Any choice is down to the individual. At the end of the day, people have nothing to say to people who choose or don't choose to have kids. It's their prerogative. You can also criticise immature people who choose to bring children in this world, not having the means to provide for a child, the discussion is endless.

It may 'seem' that way, because a few people are very vocal about their beliefs. Most people don't care a bit about what someone else does with his/her life.

Because that's your opinion and an example of exaggeration.

Because they are stupid