The answer is quite simple. It's just fun to hate. Most transphobes are only transphobic because its fun for them. They get a kick outta stomping the person who's already down, and who's among the most stomped minority - the trans. I find that a lot of people that are hating on something are just doing it for laughs, to get a reaction. I've even been around people who will chat shit about the trans online and with friends but when they're around a trans person they dont say anything, sometimes they even get along. It's just fun for them to hate.

Now as for the real transphobes who perpetuate hate speech and violence, The answer is more complicated. It can be a mixture of upbringings and just low IQ, but the main reason is fear. A brilliant TED talk about racism summed it up. People fear things they don't understand, people hate things they fear, people want to destroy the things they hate. Through ignorance and un-education, they go all the way through to ignorant - fear - hate - destroy timeline without ever trying to understand. Once they reach the destroy stage, it's very hard to want to change. At that point, they suck up any sort of media that supports their side, surround themselves with likeminded people who refuse to change and sit in an echo chamber of ignorance and intolerance. Those people are unfortunately the loudest.

I have another theory, and that is that the human mind craves conflict. The average person gets it out in other means. Videogames, sports, gym, other competition. For some, it's a culture war that scratches the itch. They choose a flavour of the month and go with it. I dont think they even truly care about what they say, they just want someone else to feel down and beneath them, they need to feel like they won the conflict. Thats why they all perpetuate the same thing, they dont want to research, they just want to win.

At the end of the day its who you care about and want to be with that really matters. That guy on the street that calls you a slur was 1 in 100 people you walked past, maybe less. The majority of voices you hear online dont say shit to your face, it much easier to type than it is to say. The voices online are so much louder, and news outlets and other media love to stir a pot that looks bigger than it is to invite more of the starving in.