Why?
Well, at the entry level, product managers are fighting to keep costs down to meet target retail price points, and that means that they have to compromise on weight. At the high-end, engineers run into the limits of materials and manufacturing to reduce weight, forcing the use of more costly alternatives.
Take for instance this alloy seatpost from a $1100 bike. It is a BRICK at 409gr. For a post more than 200gr lighter, it would be carbon fibre and cost at least $190 alone. But post 100gr lighter would have added $5-20 to the cost of the bike, which might be just enough to turn-off the price-conscience consumer, who only sees the bottom line.