Maybe because, altho it's surprising how many people do not know how to transpose, a lotta real nice and smart folks don't.
Or maybe because really good songs arrranged well often have little odd surprises in the arrangement, and the requestor wants to duplicate the sound they've heard, and they know they're off somedamnwhar. Knowing they are not perfect, they do the sensible thing-- ask someone who knows more and may be disposed to help a fellow human bean.
But really, because people are people, who consider them part of the chain of life called "Pass it on." Would you have appreciated being answered, for your query in this thread, "Why the fock can't you figger that out for yerself?" Or, were you really asking-- was it actually just a wee little pissy rant?
In any event, I hope the replies are useful to you.