[strike]Look OP. You asked a fairly open ended question, got loads of valid answers/reasons, and have responded defensively and with repeated effort to vastly limit the frame of your original post. Maybe take a step back and accept that you have your answer(s). [/strike]

Lighter DOES equal trade offs (whether you personal accept or realize that they are real), manufacturers and LBS have different realities than a home builder, and cost IS a very relevant component of your discussion topic. Sure, you got some feather-ruffling responses but you also got a ton of quality answers to your question. So what is the problem or further question? If it is solely "why don't they make lighter in the top tier bikes" you already have a bunch of perfectly correct answers. Which one comes into play depends on the manufacturer, their corp culture, and their spec decisions. Bottom line is that lower weight comes at a price for them and often at a sacrifice for their customers (not necessarily price).

EDIT: Strikethrough after finishing the last page. Seems you have dialed it back and understand that there were reasons/answers to your core question.