I think the fact that places like IKEA and Argos sell modern furniture cheaply means that many younger people would rather have something new and fashionable than an old piece - even if there is no comparison in the quality.
Furniture can be tricky to shift for the reasons already mentioned (transport being a main one - you can't put a sofa in the back of a car), and also because modern houses often have smaller rooms, and people are less likely to have designated dining rooms that can take large tables that don't fold away. The trend is for open plan living. Even things like bookcases are out of fashion, as everything gets downloaded now. We recently had a fruitless search for bookcases to line a wall, and in the end had them built for us. The only ones we could find in shops were in places like Oak Furniture Land, and they were made for lower ceilings and were more for display than books, as the shelves were fixed and too far apart for what we wanted, which was lots of book space.
Also, the 'traditional' way of setting up home on marriage is less popular too. Both partners are likely to have had homes before getting together, so will have furniture of their own, even if the homes were rented. It is a shame for the planet, but I think it's understandable that young people want to make their homes theirs, rather than in the fashion of decades earlier.