It's a feedback loop: once you begin, no matter how innocently, you start to spin uncontrollably towards becoming a woman. All it takes is a small taste to get started.
At first you're afraid of the consequences. You know that it's physically and physiologically impossible to become a woman by wearing women's clothes. Still, you proceed with caution. Nobody can find out about it. You start slow, just in case. You know for sure that some things you'd never, ever dare to do, because that would be going too far. But what's the harm in imagining it?
Now you know you want to be a woman. The idea paradoxically gives you a massive erection. You dream of sucking cock, getting fucked in the ass, and dressing like a slut at all times. As you consider transitioning and fulfilling your dream, you look back and wonder: didn't you always want to be a woman? Weren't all of those experiments over the years just your repressed femininity struggling to come to the surface? Or was it your dressing up that developed your femininity over time?
So while it is true that wearing women's clothes won't make a man physically become a woman, it certainly does affect him psychologically.