Because copying an entire chain of messages after your reply doesn't make any sense when people can scroll down to see all the messages, chained one after the other. What makes sense is to only provide the snippet that you are actually replying to. You need to do the following:
1. Select the text you are replying to in Gmail.
2. Hit the reply button.
3. Boom! Only the selected text will be quoted. Reply at will.This is common in all mail programs, but most people don't know it exists in Gmail too.
So start using it, please. Pass it around and enjoy the love of your correspondents, who will be grateful forever for your neat replies.