When I was in high school, I read about someone undergoing "sex-change" surgery. I thought I would do it if I ever won the lottery. GRS, FFS. BA, Voice surgery, the works. Get it over with quickly, heal up, and begin my new life.
When we are young, our lives are more resilient. An abrupt change like that is a minor speedbump and everything else sort of smoothes out along the way. But when we are older, we have been in relationships longer, people have become more set in their ways, and as Jess pointed out, many relationships cannot survive that.
Having lived the long-drawn-out (unnecessarily slow) version of transition has helped me to see that you can't just flip a switch and be on your way. The body needs time to adjust to hormones. It took me four years of trying different dosages, brands, and methods just to get my hormones steady in the proper range. It takes time to learn different behaviors. As a kid, I learned that to prevent being bullied, I had to become a bully. Even today, if I feel threatened, I become very aggressive. That takes time to unlearn and is one of many things that therapy is helping me with.
Would I love the snap-my-fingers-and-it's-done method? Absolutely! But I don't believe that is the best way for most people. I think that most transitions take far longer than they need to, for a variety of reasons. But I also think that it is not something that can easily be rushed. I think if it were, there would be many, many more people regretting their decision and wanting to detransition. The slow process provides time for contemplation to be certain that this is the path you want to take.