Because some Transsexual/Transgender people do not respect themselves, thus they cannot command respect from others. In a nutshell, they lack a sense of personal pride.
Words alone are harmless.
It is IDEAS that are dangerous, and some dangerous ideas are readily and intentionally expressed through certain specific words. Hatred and violence often begins with words which lead to actions, so it logically follows that we should interrupt verbal bigotry and prejudice before it escalates to physical.
Each and every time a person refers to another person by a pejorative term, they are dehumanizing that person, and each and every time we refer to ourselves this way, or allow others to do it, we are dehumanizing ourselves. After awhile, you have a person with no regard for themselves, others, or humanity in general.
So you see, it's not the word that is offensive, it's the idea that is represented by using the word that is offensive.
Have you ever had someone get angry and curse you out in a different language that you don't understand? It's an interesting experience, because while you understand the person's intent, without comprehending the actual words, it's hard to react the same as you would were you spoken to in English.
It's not just sticks and stones that hurt. Words can damage. A lot. It's just not visible damage. I taught martial arts to children for many years, and teaching them how to deal with verbal bullying is one of the most difficult challenges I faced. It's unfair to tell a child to just "ignore" the words, because they don't have an adult ego with it's accompanying confidence and self-esteem to protect them. They still don't know WHO they are, and until a person is able to feel personal pride, it's extremely difficult to evoke the "stand up for yourself" reaction.
By the time a Transsexual person completes their Transition, they will have acquired a strong sense of personal pride in their now confirmed gender (sometimes for the first time in their lives) and they will naturally want to defend this identity. It is perfectly reasonable and normal to feel offended and disrespected if someone refers to you by a term that invalidates some or all of what you worked so hard for, and achieved.
So why do some Transsexual people allow themselves to be called "tranny?" Well, here's a clue... a *complete* Transition is one wherein you correct your Transsexualism physically and mentally so that you know you are the gender you feel yourself to be, AND you also get confirmation from the world around you that you are the gender you feel yourself to be. If you don't have both, then chances are you haven't completed your Transition. When someone "settles" for an identity that is something other than "male" or "female", look carefully and see if they've completed their Transition. Do they live, work and socialize full time as a man or woman, or are they known around town as "a transsexual." Do they behave appropriately for their age and gender? Do they sound naturally male or female? Do they present themselves in a plausible, convincing fashion.
Chances are you will find that those folks who are comfortable with being called "tranny" are the same folks who, for whatever reason, found they couldn't accomplish the true goal of Transition, which is, of course, to get back the gender identity that nature denied you.
And hey, if you want to be known as a "tranny", that's fine too, but if you decide to live that life, don't expect people to see you as a "man" or "woman." They won't. You will always be a "tranny", and you will always deal with the slings and arrows that accompany a marginalized position in society. I think the vast majority of us who were born Transsexual have too much personal pride to settle for this, especially at a time in history when there are more resources than ever before available to us.
Frankly, I consider the word "tranny" to be a synonym for the word LAZY.
@ JOHN - "because I never speak to trannies, or notice if I'm spoken to by them."
Too true, but you sure love to converse with Transsexual women. I've got the emails to prove it.